INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
ENVIRONMENTAL HEALTH CRITERIA 16RADIOFREQUENCY AND MICROWAVES
This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of either the World Health Organization, United Nations
Environment Programme, or the International Radiation Protection Association.
Published under the joint sponsorship of
the United Nations Environment Programme,
World Health Organization and the International
Radiation Protection Association
World Health Organization
Geneva, 1981
ISBN 92 4 154076 1
(c) World Health Organization 1981
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CONTENTS
ENVIRONMENTAL HEALTH CRITERIA FOR RADIOFREQUENCY AND MICROWAVES
1. SUMMARY AND RECOMMENDATIONS FOR FURTHER STUDIES1.1. Summary1.1.1. Physical characteristics in relation to biological
effects
1.1.2. Sources and control of exposure1.1.3. Biological effects in experimental animals1.1.4. Power density ranges in relation to health effects1.1.5. Exposure effects in man1.1.6. Health risk evaluation as a basis for exposure
limits
1.2. Recommendations for further studies, exposure limits, and
protective measures
1.2.1. General recommendations1.2.2. Measurement techniques1.2.3. Safety procedures1.2.4. Biological investigations1.2.5. Epidemiological investigations1.2.6. Exposure limits and emission standards
1.2.6.1 Occupational exposure limits
1.2.6.2 Exposure limits for the general population
1.2.6.3 Emission standards
1.2.6.4 Implementation of standards
1.2.6.5 Other protective measures
1.2.6.6 Studies related to the establishment of
limits
2. MAGNITUDE OF EXPOSURE TO MICROWAVE AND RF RADIATION AND SOURCES
OF CONCERN
3. PROPERTIES OF MICROWAVE AND RADIOFREQUENCY (RF) RADIATION3.1. Units of radiation3.2. Other physical considerations4. SOURCES AND CONDITIONS OF EXPOSURE4.1. Natural background sources4.2. Man-made sources4.2.1. Deliberate emitters4.2.2. Sources of unintentional radiation4.3. Estimating exposure levels4.3.1. Far-field exposure4.3.2. Near-field exposure4.4. Facilities for controlled exposure4.4.1. Free space standard field method4.4.2. Guided wave methods4.4.3. Standard probe method5. MEASURING INSTRUMENTS5.1. General principles5.2. Types of instruments in common use5.2.1. Diode rectifier5.2.2. Bolometer5.2.3. Thermocouple6. MICROWAVE AND RF ENERGY ABSORPTION IN BIOLOGICAL SYSTEMS6.1. Methods of computation6.2. Experimental methods6.3. Energy absorption6.4. Molecular absorption7. BIOLOGICAL EFFECTS IN EXPERIMENTAL ANIMALS7.1. Hyperthermia and gross thermal effects7.2. Effects on the eye7.3. Neuroendocrine effects7.4. Nervous system and behavioural effects7.5. Effects on the blood forming and immunocompetent cell
systems
7.6. Genetic and other effects in cell systems7.7. Effects on reproduction and development8. HEALTH EFFECTS IN MAN8.1. Effects of occupational exposure8.1.1. Effects on the eyes8.1.2. Effects on reproduction and genetic effects8.1.3. Cardiovascular effects8.2. Medical exposure9. RATIONALES FOR MICROWAVE AND RF RADIATION PROTECTION STANDARDS9.1. Principles9.2. Group 1 standards9.3. Group 2 standards9.4. Group 3 standards9.5. RF radiation standards (100 kHz to 300 MHz)10. SAFETY PROCEDURES FOR OCCUPATIONALLY EXPOSED PERSONNEL10.1. Procedures for reducing occupational exposure11. ASSESSMENT OF DATA ON BIOLOGICAL EFFECTS AND RECOMMENDED EXPOSURE
LIMITS
REFERENCES
GLOSSARY
ANNEX
NOTE TO READERS OF THE CRITERIA DOCUMENTS
While every effort has been made to present information in the
criteria documents as accurately as possible without unduly delaying
their publication, mistakes might have occurred and are likely to
occur in the future. In the interest of all users of the environmental
health criteria documents, readers are kindly requested to communicate
any errors found to the Division of Environmental Health, World Health
Organization, Geneva, Switzerland, in order that they may be included
in corrigenda which will appear in subsequent volumes.
In addition, experts in any particular field dealt with in the
criteria documents are kindly requested to make available to the WHO
Secretariat any important published information that may have
inadvertently been omitted and which may change the evaluation of
health risks from exposure to the environmental agent under
examination, so that the information may be considered in the event of
updating and re-evaluation of the conclusions contained in the
criteria documents.
WHO/IRPA TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR
RADIOFREQUENCY AND MICROWAVES
Members
Dr V. Akimenko, A. N. Marzeev Institute of General and Communal
Hygiene, Kiev, USSR
Professor P. Czerski, National Research Institute of Mother and Child,
Warsaw, Poland (Rapporteur)a
Mme A. Duchêne, Département de Protection, Centre d'Etudes nucléaires,
Fontenay-aux-Roses, Francea
Dr M. Faber, Finsen Institute, Finsen Laboratory, Copenhagen,
Denmarka
Professor M. Grandolfo, Radiation Laboratory, Higher Institute of
Health, Rome, Italy
Mr F. Harlen, National Radiological Protection Board, Harwell, England
Dr H. Jammet, Département de Protection, Centre d'Etudes nucléaires,
Fontenay-aux-Roses, Francea
Dr J. Kupfer, Central Institute for Occupational Medicine of the GDR,
Berlin, German Democratic Republic (Vice-Chairman)
Dr M. Repacholi, Health Protection Branch, Department of National
Health and Welfare, Ottawa, Canada
Dr B. Servantie, E.A.S.S.M. -- Centre d'Etudes et de Recherche de
Biophysiologie, Toulon Naval, France
Dr M. Shore, Bureau of Radiological Health, Food and Drug
Administration, Department of Health, Education and Welfare,
Rockville, MD, USA (Chairman)a From the Committee on Non-Ionizing Radiation of the International
Radiation Protection Association
Representatives of other Organizations
Mr H. Pouliquen, International Telecommunications Union, Geneva,
Switzerland
Dr G. Verfaillie, Commission of European Communities, Brussels,
Belgium
Secretariat
Mr S. Fluss, Health and Biomedical Information Programme, WHO, Geneva,
Switzerland
Dr E. Komarov, Environmental Health Criteria and Standards, Division
of Environmental Health, WHO, Geneva, Switzerland (Secretary)
THE INTERNATIONAL SYSTEM OF UNITS (SI) COMMONLY USED IN ELECTROMAGNETICS
Name of Dimension Symbol for
of Quantity quantity SI unit and symbol
Admittance Y siemens (S)
Area. surface S square metre (m2)
Attenuation A 1 (Non-SI unit is the dB)
Attenuation coefficient alpha reciprocal metre (1/m)
Capacitance C farad (F)
Charge Q coulomb (C)
Charge, volume density of q coulomb per cubic metre (C/m3)
Conductance, electric G siemens (S)
Conductivity Â siemens per metre (S/m)
Current I ampere (A)
Current density J ampere per square metre (A/m2)
Dielectric polarization P coulomb per square metre (C/m2)
(P = D - epsilono E)
Dipole moment (electric) p coulomb œ metre (C œ m)
Dipole moment (magnetic) j weber metre (Wb œ m)
Electric field strength K volt per metre (V/m)
Electric flux phi coulomb (C)
Electric flux density D coulomb per square metre (C/m3)
Electric polarization D1 coulomb per square metre (C/m2)
Electric potential V volt (V)
Electric susceptibility chie (see glossary)
(chie = epsilonr - 1)
Energy or work E joule (J)
Energy density w joule per cubic metre (J/m3)
Frequency f hertz (Hz)
Impedance Z ohm (OMEGA)
Impedance, characteristic Zo ohm (OMEGA)
Inductance, mutual M henry (H)
(Cont'd)
Name of Dimension Symbol for
of Quantity quantity SI unit and symbol
Length I metre (m)
Magnetic field strength H ampere per metre (A/m)
Magnetic flux PHI weber (Wb)
Magnetic flux density B tesla (T)
Magnetic polarization B1 tesla (T)
Permeability µ henry per metre (H/m)
Relative permeability µr (see glossary)
(µr= µ/µo)
Permittivity epsilon farad per metre (F/m)
Phase coefficient ß radian per metre (rad/m)
Power P watt (w)
Power gain G 1 (Non-SI unit is the dB)
(G = 10 log (P2/P1))
Poynting vector S watt per square metre (W/m2)
Propagation coefficient gamma Units for alpha and ß given separately
(gamma = alpha + jß)
where alpha = attenuation
coefficient and ß =
phase coefficient
Radiant intensity I watt per steradian (W/sr)
Reactance X ohm (OMEGA)
Wavelength lambda metre (m)
a The SI units for non-ionizing radiation have not been completely developed.
Terms used in the text but not defined in this table can be found in the
glossary at the end of the document.
ENVIRONMENTAL HEALTH CRITERIA FOR RADIOFREQUENCY AND MICROWAVES
A Joint WHO/IRPA Task Group on Environmental Health Criteria for
Radiofrequency and Microwaves met in Geneva from 18-22 December 1978.
Dr B. H. Dieterich, Director, Division of Environmental Health, opened
the meeting on behalf of the Director-General. The Task Group reviewed
and revised the draft criteria document, made an evaluation of the
health risks of exposure to radiofrequency and microwaves, and
considered rationales for the development of exposure limits.
In November 1971, the WHO Regional Office for Europe convened a
Working Group meeting in The Hague which recommended, inter alia,
that the protection of man from microwave radiation hazards should be
considered a priority activity in the field of non-ionizing radiation
protection. To implement these recommendations, the Regional Office
decided to prepare the manual on "Non-Ionizing Radiation Protection",
which will include a chapter on microwave radiation (WHO, 1981).
In 1973, a symposium, sponsored by WHO and the Governments of
Poland and the USA, was held in Warsaw, on the biological effects and
health hazards of microwave radiation. This symposium provided one of
the first opportunities for an international exchange of quite diverse
opinions on the effects of microwaves. Recommendations adopted by the
symposium included the promotion and coordination, at an international
level, of research on the biological effects of microwaves, and the
development of a non-ionizing programme by an international agency
(Czerski et al., 1974a).
The International Radiation Protection Association (IRPA) became
responsible for these activities by forming a Working Group on
Non-Ionizing Radiation at its meeting in Washington, DC, in 1974. This
Working Group later became the International Non-Ionizing Radiation
Committee (IRPA/INIRC) at the IRPA meeting in Paris in 1977 (IRPA,
1977).
Two WHO Collaborating Centres, the National Research Institute of
Mother and Child, Warsaw (for Biological Effects of Non-Ionizing
Radiation) and the Bureau of Radiological Health, Rockville (for the
Standardization of Non-Ionizing Radiation) cooperated with the
IRPA/INIRC in initiating the preparation of the criteria document. The
final draft was prepared as a result of several working group
meetings, taking into account comments received from the national
focal points for the WHO Environmental Health Criteria Programme in
Australia, Canada, Japan, Netherlands, New Zealand, Poland, Sweden,
the United Kingdom, and the USA as well as from the United Nations
Environment Programme, the United Nations Industrial Development
Organization, the International Labour Organisation, the Food and
Agriculture Organization of the United Nations, the United Nations
Educational, Scientific and Cultural Organization, and the
International Atomic Energy Agency. The collaboration of these
national institutions and international organizations is gratefully
acknowledged. Without their assistance this document could not have
been completed. In particular, the Secretariat wishes to thank
Professor P. Czerski, Mrs A. Duchêne, Mr F. Harlen, Dr M. Repacholi,
and Dr M. Shore for their help in the final scientific editing of the
document.
The document is based primarily on original publications listed in
the reference section. Additional information was obtained from a
number of general reviews, monographs and proceedings of symposia
including: Gordon, 1966; Presman, 1968; Petrov, ed., 1970; Silverman
et al., ed., 1970; Marha et al., 1971; Michaelson, 1977; Minin, 1974;
Dumanski et al., 1975; Tyler, ed., 1975; Baranski & Czerski, 1976;
Glaser & Brown, 1976; Glaser et al., 1976, 1977; IVA Committee, 1976;
Johnson et al., 1976; Johnson & Shore, ed., 1977; Justesen & Guy,
1977; Hazzard, ed., 1977; Serdjuk, 1977; Taylor & Cheung, ed., 1977;
National Health and Welfare, Canada, 1977, 1978; Durney et al., 1978.
Modern advances in science and technology change man's
environment, introducing new factors which besides their intended
beneficial uses may also have untoward side effects. Both the general
public and health authorities are aware of the dangers of pollution by
chemicals, ionizing radiation, and noise, and of the need to take
appropriate steps for effective control. The increasing use of
electrical and electronic devices, including the rapid growth of
telecommunication systems (e.g., satellite systems),
radiobroadcasting, television transmitters, and radar installations
has increased the possibility of human exposure to electromagnetic
energy and, at the same time, concern about possible health effects.
This document provides information on the physical aspects of
electromagnetic fields and radiowaves in the frequency range of 100
kHz-300 GHz, which has been arbitrarily subdivided according to the
traditional approach into microwaves (300 MHz to 300 GHz) and
radiofrequencies (100 kHz to 300 MHz). A brief survey of man-made
sources is presented. It is known that electromagnetic energy in this
frequency range interacts with biological systems and a summary of
knowledge on biological effects and health aspects has been included
in the document. In a few countries, concern about occupational and
public health aspects has led to the development of radiation
protection guides and the establishment of exposure limits. Several
countries are considering the introduction of recommendations or
legislation concerned with protection against the untoward effects of
non-ionizing energy in this frequency range. In others, the tendency
is to revise existing standards and to adopt less divergent exposure
limits. It is hoped that this criteria document may provide useful
information for the development of national protection measures
against non-ionizing radiation.
Details of the WHO Environmental Health Criteria Programme,
including some of the terms frequently used in the documents, can be
found in the introduction to the environmental health criteria
document on mercury (Environmental Health Criteria 1 - Mercury, World
Health Organization, Geneva, 1976), now available as a reprint.
1. SUMMARY AND RECOMMENDATIONS FOR FURTHER STUDIES1.1 Summary1.1.1 Physical characteristics in relation to biological effects
Microwave and radiofrequency (RF) radiation constitute part of the
whole electromagnetic spectrum. This document is concerned with
frequencies lying between 105 and 3 × 1011 Hz (100 kHz and
300 GHz). The term radiofrequency refers to the range 100 kHz-300 MHz
(3 km to 1 m wavelength in air) and microwaves to the frequency range
of 300 MHz-300 GHz (1 m to 1 mm wavelength in air).
Exposure conditions in the microwave range are usually described
in terms of "power density" and are reported in most studies in watts
per square metre, or milliwatts or microwatts per square centimetre
(W/m2, mW/cm2, µW/cm2). However, close to microwave and RF
sources with longer wavelengths, the values of both the electric (V/m)
and magnetic (A/m) field strengths provide a more appropriate
description of the radiation. Exposure conditions can be altered
considerably by the presence of objects, the degree of perturbation
depending on their size, shape, orientation in the field, and
electrical properties. Very complex field distributions can occur,
both inside and outside biological systems exposed to microwaves and
RF. Refraction of the radiation within these systems can focus the
transmitted radiation resulting in markedly nonuniform fields and
energy deposition. Different energy absorption rates may result in
thermal gradients causing biological effects that may be generated
locally, difficult to predict, and perhaps unique.
When electromagnetic radiation passes from one medium to another,
it can be reflected, refracted, transmitted, or absorbed, depending on
the biological system and the frequency of the radiation. Absorbed
microwave and RF energy can be converted to other forms of energy and
cause interference with the functioning of the living system. Most of
this energy is converted into heat. However, not all microwave and RF
radiation effects can be explained in terms of the biophysical
mechanisms of energy absorption and conversion to heat. It has been
demonstrated both theoretically and experimentally that other types of
energy conversion are possible. Interactions at the microscopic level
leading to perturbations in complex macromolecular biological systems
(cell membranes, subcellular structures) have been postulated.
Biological phenomena caused by such perturbations are expected to show
a resonant frequency dependence.
1.1.2 Sources and control of exposure
General population exposure from man-made sources of microwave and
RF radiation now exceeds that from natural sources by many orders of
magnitude. The rapid proliferation of such sources and the substantial
increase in radiation levels is likely to produce "electromagnetic
pollution". Major man-made sources include: radar installations,
broadcasting and television networks, and telecommunication equipment.
In industrial, commercial, and home equipment, notably those where
energy is applied for heating purposes such as plastic sealing,
welding, drying, cooking, and defrosting, there may be extraneous
emission (leakage) of microwave or RF radiation.
The problem of this extraneous radiation or pollution from sources
of 100 kHz-300 GHz electromagnetic waves varies from country to
country, depending on the degree of industrialization. Radiation
emitted from high power sources such as broadcasting and
telecommunication networks propagates over large distances and may
even cover the whole circumference of the globe. With the increasing
use of transmitter/receivers by sea and air traffic, and the necessity
for ground-based radar control, increased levels of environmental
electromagnetic radiation may constitute a problem in many countries.
Problems of pollution range from electromagnetic interference,
particularly in relation to the operation of health services, to
direct risks to the health of individuals.
1.1.3 Biological effects in experimental animals
When sufficient microwave and RF radiation is absorbed and
converted into heat there is a consequent rise in temperature in the
organism. Injuries that have been studied in animals have resulted
from exposure to high levels of radiation and have varied from local
lesions and necrosis to gross thermal stress from hyperthermia. Death
from hyperthermia was found to occur following exposure to power
densities of a few tens of mW/cm2 to several hundreds of mW/cm2,
depending mainly on the size of the animal and the radiation
frequency. Recently, there has been a much wider appreciation of the
consequences of nonuniform energy deposition (as described in sections
6:3 and 7.1). Lesions have been found in the internal organs of
animals exposed for prolonged periods during which there was no
significant rise in rectal temperature. Furthermore, such animals did
not show any overt signs of distress.
Acute exposures may cause injury to the eye. The cornea and
crystalline lens are particularly susceptible to injury within the
frequency range of 1-300 GHz. The cornea is at greatest risk between
10 and 300 GHz and the crystalline lens from 1 to 10 GHz. For
short-terma exposures, the cataractogenic incident power density
levels lie within the range of 150-200 mW/cm2. Cataract formation
induced by a 1-h exposure can take as long as 10-14 days to develop.
The formation of retinal lesions is also possible.
It has been demonstrated that low-level,b long-terma exposure
may induce effects in the nervous, haematopoietic, and immunocompetent
cell systems of animals. Such effects have been reported in small
animals (rodents) exposed to incident power density levels as low as
0.1-1.0 mW/cm2. The reported effects on the nervous system include
behavioural, bioelectrical, metabolic, and structural (at the cellular
and subcellular levels) changes. Erythrocyte production and
haemaglobin synthesis may be impaired and immunological reactivity
changed. All these effects may influence the susceptibility of animals
to other environmental factors. For example medium level, long-term
exposure increases the sensitivity of animals to neurotropic drugs,
particularly those inducing convulsions. Thermal mechanisms seem
wholly inadequate to account for the results of studies indicating
that cerebral tissue, exposed to weak electromagnetic fields, responds
only over a limited range of intensities and modulation frequencies of
the RF carrier field. There appears to be evidence for both amplitude
and modulation frequency windows, outside which effects are not
observed.
Genetic effects, effects on development, and teratogenic effects
have been observed in animals and plants. Numerous reports have
indicated that at sufficiently high intensities, microwave and RF
exposure may induce chromosomal aberrations, and also disturbances in
somatic cell division (mitosis), germ cell maturation (meiosis), and
spermatogenesis (section 7.7). The intensity levels required to
produce these effects seem to indicate that a thermal mechanism may be
responsible. Existing information on the influence of microwave and RF
exposure on the transmission and expression of hereditary traits is,
however, insufficient. No threshold levels or dose-effect
relationships can be established at present.
1.1.4 Power density ranges in relation to health effects
During the 1973 Warsaw International Symposium on biological
effects and health hazards of microwave radiation, it was agreed that
microwave power densities could be divided into ranges. The following
is an abridged version of this agreement:
Microwave densities may be divided into the following 3 ranges:
(a) High power densities, generally greater than 10 mW/cm2, at
which distinct thermal effects (see Glossary) predominate;
a See Glossary.
b In this document, the ranges for low, medium, and high level
exposures are approximately those agreed by the Warsaw symposium
(section 1.1.4).
(b) Medium power densities, between 1-10 mW/cm2, where weak but
noticeable thermal effects exist; and
(c) Low power densities, below 1 mW/cm2, where thermal effects
are improbable, or at least do not predominate.
The boundaries indicated for these ranges are arbitrary and depend
on numerous factors, such as animal size, threshold of warmth
sensation, frequency, and pulsing. The introduction of the
intermediate range of subtle effects calls attention to the need for
additional research, aimed at clarification of the underlying
mechanisms.
It should be noted that the classification applied to the
microwave region (300 MHz-300 GHz). A similar classification was not
determined for the RF region (100 kHz-300 MHz).
1.1.5 Exposure effects in man
The meagre evidence available on exposure effects in man has been
obtained from incidents of accidental acute over-exposure to
microwaves and RF. Not enough attention has been given to the conduct
of epidemiological investigations. In some human studies, which have
been conducted on people exposed occupationally, subjective symptoms
have been reported.
A considerable number of people in many countries have received
microwave and RF diathermy treatment at power levels of several tens
of watts for a duration of about 20 min daily over a period of some
weeks.
Adverse effects have not been adequately investigated among
diathermy patients. This is a group of people exposed to microwaves
and RF who can be readily identified and such studies should be
carried out, as they may yield considerable information concerning
exposure effects in man.
1.1.6 Health risk evaluation as a basis for exposure limits
Theoretical considerations, experimental animal studies, and
limited human occupational exposure data constitute the basis for the
establishment of health protection standards. It should be noted that,
in some countries, microwave and RF health protection standards have
recently been changed and that there is a tendency to adopt less
divergent exposure limits in comparison with those proposed two
decades ago.
In establishing health protection standards, different approaches
and philosophies have been adopted.
A highly conservative approach would be to keep exposure limits
close to natural background levels. However, this is not technically
feasible. A reasonable risk-benefit analysis has to be considered.
More data on the relationship between biological and health
effects and the frequency and mode of generation of the radiation,
particularly in complex modulations, are needed.
In the case of pulse modulation, peak power density may be a
factor which should be considered in setting exposure limits. However,
it is not possible to propose a limit of peak power density from the
information available at present.
1.2 Recommendations for Further Studies, Exposure Limits, and
Protective Measures1.2.1 General recommendations
The basic biophysical mechanisms of interaction of microwaves and
RF with living systems still need clarification and further studies.
Work on both theoretical and experimental dosimetry, the
calculation and measurement of fields and of energy deposited within
simulated or actual biological systems, should be continued and
refined.
Results of animal studies are difficult to extrapolate to man, and
these studies alone do not constitute a satisfactory basis for the
establishment of health protection criteria. They should, therefore,
be supplemented by appropriate epidemiological studies in man.
The existing data on power, amplitude, and frequency "windows"
seem to warrant continued investigations.
The effects of chronic exposure on sensitivity to convulsant and
other drugs are potentially useful and may have a direct bearing on
the development of exposure standards.
Long-term, low-level exposures combined with such stresses as high
ambient temperature and humidity should be investigated.
There is little published information on dose-effect
relationships; reports tend to be limited to whether effects are
observed at one particular level of exposure rather than over a range.
More dose-related information, even covering small subject areas,
would be valuable.
Investigations on the genetic effects and effects on development
of microwave and RF radiation should have priority.
Attention should be given to investigating the different
sensitivities to microwave/RF exposure of subgroups within the general
population.
National and international agreements on exposure limits, ways and
means of controlling this type of environmental pollution, and
concerted efforts to implement such agreements are needed.
1.2.2 Measurement techniques
There is a continuing need for the development of microwave/RF
measuring instruments that: (a) give direct readings of electric or
magnetic field strength, or power density; (b) are robust; (c) are
portable, light-weight, and battery-operated; and (d) are sensitive
and can be used over a wide frequency range.
The problem of the design of personal dosimeters also remains to
be solved.
1.2.3 Safety procedures
Computation techniques or methods that predict the distribution of
fields close to deliberate high-power emitters (in the near field) are
needed.
Emphasis should be placed on the development of technology to
ensure containment and limitation of radiation to the deliberately
exposed object, as well as the reduction of leakage emission from
devices.
Personal protective devices should be used only as a last resort.
Adequate medical surveillance of occupationally-exposed persons
should be provided.
Once exposure limits have been set, safety guidelines or codes of
practice concerning safe use and installation design should be
developed as soon as possible.
1.2.4 Biological investigations
Reports of experimental work should contain sufficient information
describing the exposure conditions to allow an estimation not only of
the total absorbed energy but also, as far as possible, of the
distribution of the energy deposited within the irradiated biological
system.
Systematic investigation of the effects of microwave/RF exposure
at all levels of biological organization are to be encouraged. This
includes effects at the molecular level on subcellular components;
cells, viruses, and bacteria; organs and tissues; and whole animals.
Particular attention should be paid to: (a) long-term, low-level
exposures and possible delayed effects; (b) the possibility of
differences in sensitivity of various body organs and systems, where
specific effects in various animal species are being considered; and
(c) the influence of microwave/RF exposure on the course of various
diseases, including any possible increase in sensitivity to
microwaves/RF that may result because of the disease state.
1.2.5 Epidemiological investigations
Epidemiological studies should be carried out in a careful manner,
paying attention to the relationship between exposure to microwaves/RF
and other environmental factors occurring in the place of work and to
the health status of the investigated group. Specific biological
endpoints should be selected and adequate examination methods used for
such studies. Conventional medical examinations will not provide
sufficient information.
Studies should be carried out on (a) workers occupationally
exposed to microwave/RF sources; (b) patients treated with microwave
and RF diathermy; and (c) groups within the general population living
near high-power microwave/RF sources.
A distinction should be made between occupational and public
health protection standards.
1.2.6 Exposure limits and emission standards
1.2.6.1 Occupational exposure limits
The occupationally-exposed population consists of healthy adults
exposed under controlled conditions, who are aware of the occupational
risk. The exposure of this population should be monitored.
It is possible to indicate exposure limits from available
information on biological effects, health effects, and risk
evaluation. For occupational exposure, values within the range
0.1-1 mW/cm2 include a high enough safety factor to allow continuous
exposure to any part of the frequency range over the whole working
day. Higher exposures may be permissible over part of the frequency
range and for intermittent or occasional exposures. Special
considerations may be indicated in the case of pregnant women.
1.2.6.2 Exposure limits for the general population
The general population includes persons of different age groups
and different states of health, including pregnant women. The
possibility that the developing fetus could be particularly
susceptible to microwave/RF exposure deserves special consideration.
Exposure of the general population should be kept as low as
readily achievable and exposure limits should generally be lower than
those for occupational exposure.
1.2.6.3 Emission standards
Emission standards for equipment should be derived from, and be
lower than exposure limits, where this can reasonably be achieved. A
class of equipment may be considered safe and exempt from regulations,
if hazardous levels of radiation exposure cannot originate from such a
source.
1.2.6.4 Implementation of standards
The implementation of microwave and RF occupational and public
health protection standards necessitates: the allocation of
responsibility for measurements of radiation intensity and
interpretation of results; and the establishment of detailed radiation
protection safety codes and guides for safe use, which indicate, where
appropriate, ways and means of reducing exposure.
1.2.6.5 Other protective measures
Prevention of health hazards related to microwave and RF radiation
also necessitates the establishment of rules for the prevention of
interference with medical electronic equipment and devices such as
cardiac pacemakers, prevention of detonation of electroexplosive
devices, and prevention of fires and explosions due to the ignition of
flammable material (vapours) by sparks originating from induced
fields.
1.2.6.6 Studies related to the establishment of limits
Studies of the frequency and modulation dependence of biological
and health effects are of prime importance. The results of such
investigations may make it possible to modify the rationales of
present day standards and to identify frequencies at which exposure
limits should be lower or higher than those suggested in section 11.
2. MAGNITUDE OF EXPOSURE TO MICROWAVE AND RF RADIATION AND SOURCES OF
CONCERN
Electromagnetic fields and RF radiation occur naturally over a
very wide range of frequencies. The ionosphere very effectively
shields the earth's biosphere from radiations of this type originating
in space. Electromagnetic fields and radiation of high intensity may
be generated by natural electrical phenomena such as those
accompanying thunderstorms.
However, in the frequency range of 100 kHz to 300 GHz, the
intensity of natural fields and radiation is low. Exposure of the
urban population in the USA to man-made microwave sources was found by
Janes (1979) to vary from a very low value to as high as 100 µW/cm2.
The median exposure to the total microwave flux from external sources
for this population was calculated to be 0.005 µW/cm2. Osepchuk
(1979) has calculated the background exposure from the sun, integrated
up to 300 GHz to be 1.4 × 10-5 µW/cm2. These values can be put in
better perspective by noting that the integrated microwave/RF flux
emitted from the human body has been calculated by Justesen (1979) to
be up to 0.5 µW/cm2.
The proliferation of man-made sources of energy in the 100 kHz-300
GHz range has only occurred over the last few decades. From the point
of view of biological evolution, this energy constitutes a very recent
physical factor in the environment. Observations of biological effects
from exposure to microwaves gave rise to concern in the early 1940s.
On the basis of special research programmes, radiation protection
guides recommending exposure limits were developed in the 1950s in the
USSR and the USA. Thereafter, several industrialized countries
introduced recommendations and/or legislation on microwave and RF
health protection. It should be noted, however, that exposure limits
vary widely, and are the subject of many discussions and much
controversy.
Although concern about microwave and RF effects and possible
hazards arose first in highly developed countries, the problem is
universal. Developing countries are rapidly establishing
telecommunications, broadcasting systems, and other sources of
electromagnetic energy. Electromagnetic waves emitted in particular
countries may propagate around the globe. A report from the USA
(Office of Telecommunications Policy, 1974) states: "Unless adequate
monitoring programs and methods of control are instituted in the near
future, man may soon enter an era of energy pollution comparable to
that of chemical pollution of today."
The urgent need for international agreement on maximum exposure
limits and international programmes for the containment of
electromagnetic pollution has been stressed at international meetings
(Czerski et al., 1974a). Prevention of potential hazards is a more
efficient and economical way of achieving control than belated efforts
to reduce existing levels.
3. PROPERTIES OF MICROWAVE AND RADIOFREQUENCY (RF) RADIATION
Radiowaves in the frequency range 100 kHz-300 GHz are non-ionizing
electromagnetic radiation, and can be described in terms of
time-varying electric and magnetic fields moving though space in
wavelike patterns, as represented in Fig. 1.
The wavelength (the distance between corresponding points of
successive waves) and the frequency (the number of waves that pass a
given point in 1 second) are related and determine the characteristics
of electromagnetic radiation. The shorter the wavelength, the higher
the frequency. At a given frequency, the wavelength depends on the
velocity of propagation and therefore, will also depend on the
properties of the medium through which the radiation passes. The
wavelength normally quoted is that in a vacuum or air, the difference
being insignificant. However, the wavelength can change significantly
when the wave passes through other media. The linking parameter with
frequency is the velocity of light (3 × 108 m/second in air). The
velocity decreases and the wavelengths become correspondingly shorter,
when microwaves and RF radiation enter biological media, especially
those containing a large proportion of water.
Another related property of electromagnetic waves is the photon
energy, which increases linearly as the frequency increases. Fig. 2
shows the spectrum of electromagnetic radiation ranging from highly
energetic ionizing radiation with extremely high frequencies and short
wavelengths to the less energetic non-ionizing radiation with the much
lower frequencies and longer wavelengths of radio-frequencies.
Conventionally a photon energy of 12eV, corresponding to a
wavelength of 100 nm, is taken as the dividing line between ionizing
and non-ionizing radiation. This is in the vacuum region of the
ultraviolet spectrum. Microwave and RF radiations are much less
energetic. Their energy per photon corresponds to 1.25 × 10-3 eV at
300 GHz and 4.1 × 10-10 eV at 100 kHz, and is much too low to cause
ionization.
3.1 Units of Radiation
When microwave or RF radiation is absorbed in a medium, the most
obvious effect is heating. The radiation intensity can be determined
calorimetrically. In SI terminology, it is known as the irradiance and
is expressed in W/m2. Traditionally, however, the term "power
density" has been and continues to be used for this part of the
frequency range, and will be used in this document with the more
commonly reported units of mW/cm2 and µW/cm2.
The associated electric and magnetic field strengths (E and H)
can be equally valid expressions of radiant energy flow. When these
are stated in V/m and A/m, respectively, their product yields VA/m2.
At distances greater than about one wavelength from the source,
E and H are in phase and VA/m2 may be expressed as W/m2.
Ideally, at a distance sufficiently remote from the source of
radiation that it can be regarded as a point source, an inverse square
law of power density with distance applies, the ratio E/H is 120 pi,
i.e., 377OMEGA. The power density can, therefore, be derived from
E 2/377 or from H 2 × 377. Where E and H are expressed in
V/m and A/m (Table 1), this is referred to as plane-wave or far-field
conditions and to obtain a measure of the radiated power density, only
the E field or the H field need be measured. Most instruments used
for measuring power density measure the E field, because this
technique is more versatile and presents fewer practical problems. H
field detectors have been devised for a limited range of frequencies.
Instruments combining both types of detection are possible, in
principle, but would be most difficult to construct.
Table 1. Comparison of power densities in the more commonly used
units for free-space, far-field conditions
W/m2 mW/cm2 µW/cm2 V/m A/m
10-2 10-3 1 2 5 x 10-3
10-1 10-2 10 6 1.5 x 10-2
1 10-1 102 2 x 10 5 x 10-2
10 1 103 6 x 10 1.5 x 10-1
102 10 104 2 x 102 5 x 10-1
103 102 105 6 x 102 1.5
104 103 106 2 x 103 5
The distance beyond which far-field conditions apply is usually
taken as being 2 a 2/lambda, where a is the maximum dimension of
the source (antenna) and lambda is the wavelength. The radiated "near
field" includes distances of less than 2 a 2/lambda, where the
inverse square law with distance does not apply and the impedance in
space (the ratio E/H) may differ from 377OMEGA. Close to the source,
at distances less than lambda, reactive components of E and H
become progressively more important. Instruments, calibrated in units
of power density but based on the measurement of E, for instance,
will become increasingly inaccurate at close range. The instruments
make valid measurements of the E fields, but their scale indications
in terms of power density no longer apply. These and allied
considerations are also discussed in section 4.3.2.
3.2 Other Physical Considerations
A detailed analysis and interpretation of the perturbing effects
of objects placed in the path of microwave or radiofrequency beams
requires the solution of Maxwell's field equationsa for the
appropriate boundary conditions. However, an important insight can be
obtained by comparison with the shorter wavelength visible radiation,
to which the same equations apply. The general laws of geometrical and
physical optics remain valid: particularly the latter because of the
wavelengths involved and because deliberate generators of microwaves
and RF emit coherent radiation, i.e., the wave fronts are regular and
radiated over a narrow band of frequencies at any one time. Radiation
a A set of four fundamental equations that describe all electric
and magnetic fields. Their solution for real materials requires
knowledge of the macroscopic electrical and magnetic properties of
the materials.
reflected into the path of the incident beam will form standing waves
and, at a distance of a few wavelengths, diffraction effects can cause
additive and subtractive interference. Both effects have been
convincingly demonstrated by Beischer & Reno (1977) at 1 GHz with man
as the perturbing influence. Diffraction and internal reflections can
also take place when radiation penetrates heterogeneous materials such
as body tissues, leading to markedly nonuniform internal fields and
energy deposition. As in geometric optics, the combination of a high
refractive index and convex body contours behaves like a strong convex
lens focusing the penetrating radiation. Absorption and internal
scattering will limit the extent of these effects. Without the
absorption of energy to initiate some change, there cannot be any
biological effects.
Direct radiation is usually polarized, i.e., the E and H
fields are oriented parallel to particular orthogonal planes or rotate
in an ordered fashion. The plane of polarization of the reflected
radiation will, thus, be changed complicating the measurement of the
combined beams and investigation of the biological effects.
Orientation with respect to the plane of polarization is important in
some measuring instruments and in the distribution and total energy
absorption in animals and in man. In some radar applications, typical
equipment may emit pulses of 1 microsecond (µs) with a pause between
pulses of 1 millisecond (ms). This constitutes a factor of 103 in
the values of instantaneous power radiated and deposited, and of 30 in
the electric fields, compared with continuous wave (cw) generation at
the same average power. Thus, instruments to be used in pulsed fields
must have a wider dynamic range and more robust burn-out
characteristics.
4. SOURCES AND CONDITIONS OF EXPOSURE4.1 Natural Background Sources
Microwave and RF radiation occurs naturally, but the intensity of
natural radiation in the range of 100 kHz-300 GHz is very low in
comparison with the overall intensity of man-made radiation in this
range, as shown in Fig. 3. The intensity of natural fields is mostly
due to atmospheric electricity, which is static and has an electric
field intensity of about 100 V/m (IVA Committee, 1976). This is known
as the earth's electric and magnetic field. Radio emissions of the sun
and stars, which are equivalent to about 10 pW/cm2 in the range of
100 kHz to 300 GHz also contribute to natural radiation. Local
disturbances leading to increased field intensities occur during
thunderstorms. Electromagnetic fields with a very wide frequency range
are created (atmospheric noise) with a maximum field intensity at
about 10 kHz (Minin, 1974; IVA Committee, 1976).
Artificial microwave and RF radiation constitutes a very recent
environmental factor, dating back only a few decades. Depending on the
frequency range, exposure from man-made sources of microwave and RF
radiation may be many orders of magnitude higher than that from
natural radiation and man as a species has had no opportunity to adapt
to microwave and RF radiation at such environmental levels (Presman,
1968).
There exists a great diversity of man-made sources, both in
respect of power output and the power densities that are generated,
and the frequency range in which the sources operate. According to the
use of the source, different segments of the general population are
exposed in different ways. There are obvious differences, depending on
the development of the country, between the average exposure of the
general population, the exposure of inhabitants of urban
industrialized areas, and the exposure of inhabitants of rural areas.
There is also a risk of exposure to microwaves and RF in some
occupations. In view of this, the discussion of man-made sources must
include a description of exposure situations.
4.2 Man-Made Sources
Any appliance that generates electricity or is driven by an
electric current generates electromagnetic fields. These propagate
through space in the form of electromagnetic waves. Man-made microwave
and RF sources may be broadly divided into 2 classes, i.e., deliberate
emitters, and sources of unintentional, incidental radiation.
4.2.1 Deliberate emitters
Deliberate emitters generally have a radiating element (antenna)
designed to emit electromagnetic waves into the surrounding
environment in a specified manner. The frequency, direction of
propagation, and the point of origin are determined by the intended
use of the equipment. Because of physical laws, and, in spite of the
degree of perfection of the design of a deliberate emitter, some
unintentional leakage, or stray radiation is always generated. This
should be taken into account when evaluating a deliberate emitter as a
radiation source.
Unintentional radiation may occur in the form of broad-band noise
or may be generated as discrete harmonics. In some instances, it is
generated by sources that emit radiation outside the microwave and RF
ranges. For example, while the intentional radiation of fluorescent
light tubes lies in the visible light range, such tubes also generate
very low levels of microwave and RF white noise (Mumford, 1949).
Typical deliberate emitters include radiobroadcasting and
television stations, radar installations, and electronic wireless
communication systems. These sources can be classified in different
ways and classifications may vary from country to country depending on
attitudes towards possible environmental and health effects. When
classified according to the nominal power output or the effective
radiated power (ERP), such emitters may be divided into high, medium,
and low power sources. Radar systems used for tracking and guiding
purposes, as well as sources used in satellite systems are among the
most powerful. It was reported in 1974 in the USA, that there were 20
nonpulsed unclassified sources with average effective radiated power
(ERP) ranging from 5 GW to 31.6 GW and one experimental source with an
average ERP of 3.2 TW (3.2 × 1012W) (Hankin, 1974). All these
sources were used in conjunction with satellite systems. A further 144
sources had an average ERP of 1 MW or more. The twenty most powerful
unclassified pulsed (radar) sources had average ERPs between 8.7 MW
and 840 MW and peak ERPs ranging from 35.4 GW to 2.8 TW; 229
unclassified pulsed sources had peak ERPs of 10 GW or more. This may
be compared with television or amplitude modulation (AM) broadcasting
stations in which the power of the transmitters is of the order of
tens of kW (usually about 50 kW) or radio telephones (walkietalkies),
in which ERPs may be of the order of a few watts or less.
Another approach towards classification of sources is to examine
the configuration of the radiated fields and their propagation in
space. Directional radiating elements (antennae) generating intense
focused beams and multidirectional, variously polarized antennae may
be used. Taking into account the power of the transmitter and the type
of the radiating element, the magnitude of distances (or zones) at
which various intensities of radiation (power densities on strength of
E or H fields) occur can be computed. In this case, the
classification of sources also depends on arbitrarily chosen levels of
radiation intensity. This approach may be useful in the siting of
sources and in establishing "safe", "hazardous", and "danger" zones
around a source.
Deliberate emitters may be also classified according to the mode
of generation. Microwaves and RF may be generated continuously or in
pulses and both continuous and pulsed wave generators may operate for
long periods (up to 24 h per day) or short intermittent periods. The
generated radiosignal may be frequency, amplitude, or pulse-modulated.
Sources with moving directional antennae and sources generating mobile
narrow beams may illuminate a point in space intermittently with a
time-varying intensity ranging from zero to extremely high, at pulse
peak power. Because of these complexities and since a point in space
may be illuminated by radiation originating from several sources, the
determination of the total or average quantity of energy delivered at
this point during a period of time, may be difficult and may
necessitate the use of sophisticated equipment and advanced computing
methods.
Evaluations of the intensity of microwave and RF radiation
generated by deliberate emitters have been published in the USA (Smith
& Brown, 1971; Tell, 1972; US Environmental Protection Agency, 1973;
Tell & Nelson, 1974a; Tell et al., 1974; Hankin et al, 1976; Stuchly,
1977; Tell & Janes, 1977; Tell & Mantiply, 1978). Fig. 4-7 illustrate
the number of sources operating in the frequency range 10 kHz-300 GHz
and capable of producing power densities equal to or greater than
10 mW/cm2, 1 mW/cm2, 0.1 mW/cm2, and 0.01 mW/cm2. These data
should be compared with data in Tables 2-6.
Table 2 presents anticipated characteristics of satellite
communications systems, which are among the most powerful sources of
continuous wave radiations (100 W/m2, 10 W/m2, 1 W/m2, and
0.1 W/m2). Table 3 presents characteristics of pulsed wave, high
power generators, and Tables 4 and 5 include characteristics of
on-board aircraft and marine radars, respectively. Table 6 shows the
characteristics of some North American broadcasting transmitters, the
electric field intensity, and the equivalent power density at ground
level at a distance of one mile from the transmitter tower. Fig. 8
presents power density versus distance for a television transmitter.
In this context, it should be stressed that according to data of the
US Environmental Protection Agency (Tell, 1972; Hankin et al., 1976;
Tell & Mantiply, 1978) and the US Bureau of Radiological Health (Smith
& Brown, 1971), broadcasting stations are "significant sources of RF
exposure" (Tell & Janes, 1977). In view of the increasing popularity
of mobile (portable or mounted on vehicles) transmitters for personal
use, field intensities in the vicinity of antennae of these citizen
band (CB) transmitters may be of concern in some countries from the
point of view of population exposure (Neuksman, 1978; Ruggera, 1979).
Table 2. Anticipated characteristics of selected satellite communication systemsa
Distance in km from antenna
for power densities of
System f(GHz) Pav(kW) Pmax(mW/cm2) 0.1 mW/cm2 1 mW/cm2 10 mW/cm2
LET 8.1 2.5 30.4 0.246 0.78 2.46
AN/TSC-54 8.1 8 50.8 0.46 1.45 4.58
AN/FSC-9 8.1 20 7.6 6.23 19.7 62.3
Intensat 6.25 5 0.73 -- -- 12.3
Goldstone Venus 2.38 450 97.3 4.16 13.2 41.6
Goldstone Mars 2.38 450 16.8 9.68 33.4 106
a From: National Health and Welfare, Canada (1977) based on Hankin et al. (1976).
Table 3. Anticipated characteristics of typical high peak power radarsa
Distance in km from antenna
for power densities of
System f(GHz) P(kW) Pmax(mW/cm2) 10 mW/cm2 1 mW/cm2 0.1 mW/cm2
Acquisition radar
FPN -- 40 9.0 0.18 12.8 0.028 0.111 0.351
Acquisition radar
ARSR 1.335 20 111 0.147 0.465 1.47
Tracking radar
Hawk Hi Power 9.8 4.7 800 0.108 0.344 1.08
Tracking radar
no. 1 2.85 12 34.2 9.392 1.24 3.93
Tracking radar
no. 2 1.30 150 55.7 1.75 5.52 17.5
a From: National Health and Welfare, Canada (1977) based on Hankin et al. (1976).
Table 4. Experimental data for typical on-board aircraft radarsa
Approximate distance from
Power Power radome in m for power
Radar Aircraft f(GHz) average density density of
system (w) max.
(mw/cm2) 10 mW/cm2 1 mW/cm2
WP 103 BAC 111 9.375 26 20 3 11
AVQ 20 Convair 9.375 16 10 2 11
AVQ 50 Convair 580 9.375 16 26 2 11
AVQ 20 DC-9 9.375 28 15 4 13
a From: National Health and Welfare, Canada (1977) based on Tell & Nelson (1974a).
Table 5. Power density in the vicinity of on-board marine radars (non-rotating antennae)a
Distance Average power density
Power from (µW/cm2)
System f(GHz) Peak Average antenna
(kW) (w) (m)
Decca 101 9.445 3 3.25 25.5 6.8 7.5 ± 5.4
Decca 202 9.445 3 1.5 45.7 3.6 5.1 ± 4.6
Decca RM316 9.41 10 5 103.6 3.7 5.9 ± 5.0
Kelvin-Hughes 17 9.445 3 2.75 103.6 0.6 1.4
Konel KRA 221 9.375 10 4.8 45.7 9.2 6.1 ± 4.5
a From: National Health and Welfare, Canada (1977), based on Peak at al. (1975).
Table 6. Parameters of broadcasting transmittersa
Frequency Maximum Tower Field Power
Service (MHz) ERP Height intensity density
(kW) (m) (mV/m) (µW/cm2)
FM Radio 88--108 100 152 1023 2.78
VHF-TV, ch. 2--6 54--88 100 305 807 1.73
VHF-TV, ch. 7--13 174--216 316 305 191 0.1
UHF-TV 470--890 5000 305 380 0.38
a From: National Health and Welfare, Canada (1977), based on Tell (1972).
Medical microwave and RF equipment (chiefly medical diathermy) is
a particular class of deliberate emitters designed and used for the
irradiation of human subjects to obtain beneficial effects. In this
case, the intended human exposure is carried out under professional
supervision and constitutes part of medical practice. The contribution
of medical uses to the general population exposure is difficult to
evaluate and varies from country to country. A survey in Pinellas
County (Florida, USA) revealed that among a population of 500 000
persons, 7037 individuals received 45 000 microwave or shortwave
diathermy treatments of various durations and exposure levels (Remark,
1971). It should be pointed out that the county has a large population
of retired people.
Although individual patients may absorb large quantities of
energy, the exposure is limited to selected body areas and limited in
time. However, medical microwave and RF equipment is also a source of
unintentional radiation (Bassen et al, 1979) and during irradiation
sessions, considerable scattering of electromagnetic fields may occur
(Witters & Kantor, 1978; Bassen et al., 1979). Thus, particular
attention should be paid to limiting exposure to the areas intended
and to avoiding additional radiation doses to the patient from
adjacent sources (other diathermy equipment). The occupational
exposure of personnel operating the equipment should also be limited.
The unintentional exposure of both patient and personnel usually
involves the whole body.
4.2.2 Sources of unintentional radiation
Electrical and electronic, industrial or commercial equipment and
consumer products in which, by design, the electromagnetic energy is
contained within a restricted area, but into which objects to be
processed are introduced, can all be sources of unintentional
radiation. Any energy (radiation) emanating outside the area
represents an energy loss. However, complete containment of
electromagnetic energy is not always technically feasible. A typical
example of a source of unintentional or leakage radiation is the
microwave oven for commercial or home use. The microwave energy should
be totally contained in the oven's cavity and used for heating
(cooking) food. Leakage of microwaves does not serve any purpose and,
if excessive, may represent a hazard to the user.
Microwave and RF equipment is used in many industries for such
processes as melting, welding, drying, gluing, plastic processing, and
sterilization. Surveys of dielectric radiofrequency heaters in Canada
(Stuchly et al., 1980; National Health and Welfare, Canada, 1980) have
shown that these heaters are used predominantly for plastic sealing
and wood gluing, and operate at frequencies between 4 and 51 MHz with
output powers in the range of 0.5-90 kW. Operators of some of these
devices were exposed to fields with equivalent power densities
exceeding 10 mW/cm2. Most industries use electrical and electronic
equipment (NIOSH, 1973). Table 7 represents various uses of microwave
and RF generating equipment within certain frequency bands. Table 8
shows frequencies allocated for industrial, scientific, and medical
uses (ISM bands) and Table 9, the frequencies allocated for these
purposes in the USA and the USSR.
Unintentional exposure to microwave and RF radiations from
deliberate emitters occurs universally. The results of a series of
investigations by the US Environmental Protection Agency (section
4.2.1) indicate that urban populations in highly industrialized
countries may be exposed to overall intensities of the order of
µW/cm2 (Gordon, 1966; Marha et al., 1971; Minin, 1974; Dumanski et
al., 1975; Baranski & Czerski, 1976; Johnson et al., 1976; IVA
Committee, 1976; National Health and Welfare, Canada, 1977, 1978;
Durney et al., 1978). Inhabitants of high buildings in the vicinity of
the rooftop antennae of broadcasting and television stations may be
exposed to levels ranging from a few hundred µW to a few mW per cm2.
According to Tell & Mantiply (1978), 50% of the urban population of
the USA is exposed to less than 0.005 mW/cm2, 95% to less than 0.01
mW/cm2, and 99% to less than 0.1 mW/cm2.
Table 7. Selected examples of the typical uses of equipment generating radiofrequency
and microwave radiation
Frequency Use Occupational exposure
Below 3 MHz Metallurgy: eddy current melting, Metal workers; radiotransmitter
tempering; broadcasting, personnel.
radiocommunications, radio-
navigation.
3--30 MHz Many industries such as the Various factory workers, e.g.,
car, wood, chemical, and food furniture veneering operators,
industries for heating, plastic sealer operators, drug &
drying, welding, gluing, food sterilizers, car industry
polymerization, and sterilization of workers; medical personnel;
dielectrics; agriculture; food broadcasting transmitter and
processing; medicine; radio- television personnel.
astronomy; broadcasting.
30--300 MHz Many industries as above; medicine; Various factory workers, as above;
broadcasting, television, medical personnel; broadcasting
air traffic control, radar transmitter and television
radionavigation. personnel.
300--3000 MHz TV, radar (troposcatter and Microwave testers; diathermy and
meteorological); microwave microwave diathermy operators
point-to-point; telecommunication and maintenance workers; medical
telemetry; medicine; microwave personnel; broadcasting transmitter
ovens; food industry; plastic and television personnel;
preheating. electronic engineers and technician:
air crews; missile launchers;
radar mechanics and operators
and maintenance workers.
3--30 GHz Altimeters; air- and ship-borne Scientists including physicists;
radar; navigation; satellite microwave development workers;
communication microwave point-to- radar operators; marine and
point. coastguard personnel; sailors,
fishermen and persons working on
board ships.
30-300 GHz Radiometeorology; space Scientists including physicists;
research; nuclear physics and microwave development workers;
techniques; radio spectroscopy. radar operators.
Table 8. Designation and use of microwave and RF Bands
Letter designation of microwave frequency Some industrial, scientific, and medical
bands (ISM) frequency bands, (not applicable
in all countries)
Band Frequency -- MHZ
L 1100-- 1700 13.56 MHz ± 6.78 kHz
LS 1700-- 2600 27.12 MHz ± 160 kHz
S 2600-- 3950 40.68 MHz ± 20 kHz
C 3950-- 5850 433 MHz ± 15 MHz
XN 5850-- 8200 915 MHz ± 25 MHz
X 8200--12 400 2450 MHz ± 50 MHz
Ku 12 400--18 000 5800 MHz ± 75 MHz
K 18 000--26 500 22 125 MHz ± 125 MHz
Ka 26 500--40 000
General population exposure may be considered as long-term, very
low-level, intermittent exposure for 24 h per day (or for major
portions of the day) to a very wide range of microwave and RF
radiation frequencies.
4.3 Estimating Exposure Levels4.3.1 Far-field exposure
Estimates of far-field exposures are necessary before powerful and
complex installations are constructed. The subject is discussed at
length by Minin (1974), who not only considers factors connected with
equipment and the local topography but also gives information on
methods of screening. Whenever possible, estimates of radiation fields
should be made before detailed surveys of potentially hazardous
exposures are carried out. Mumford (1961) gives approximate formulae
for some radar antennae; additional information can be obtained from
textbooks and monographs (Kulinkovskaja, 1970; ANSI, 1973; US
Department of Commerce, 1976; National Health and Welfare, Canada,
1977; Krylov & Jucenkova, 1979). This procedure is necessary not only
to select a suitable survey instrument but also to determine if
potentially hazardous exposure of the operator could occur, if the
instrument were faulty. Unlike instruments for ionizing radiation,
there are no sources readily available for checking the calibration of
the instrument.
Table 9. Radiofrequency and microwave band designations
Band designations
USA USSR
(a) Radiofrequency bends
Low frequency (LF) Long VCh 104-103 m 30-3 kHz radionavigation; radio
beacon AM broadcast.
Medium Medium (HF) 103-102 m 0.3-3 MHz marine radiotelephone;
frequency (MF) AM broadcasting.
High frequency (HF) Short UHF 102-10 m 3-30 MHz amateur radio; citizens band
in the USA. etc.; world-wide
broadcasting; medical diathermy;
RF sealers, welders, heaters;
short-wave diathermy.
Very high Ultra-short 10-1 m 30-300 MHz Frequence modulated (FM)
frequency (VHF) (metre) broadcasting; television; air
traffic control; radionavigation.
(b) Microwave bands
Ultra high Decimetre Super 0.3-3 GHz microwave diathermy; television;
frequency (UHF) HF microwave point-to-point;
microwave ovens & heaters;
telemetry; tropo scatter &
meteorological radar.
Table 9 (Cont'd)
Band designations
USA USSR
Super high Centimetre (SHF) 10-1 cm 3-30 GHz satellite communication; air-
frequency (SHF) borne weather radar; altimeters;
shipborne navigational radar;
microwave point-to-point;
amateur radio.
Extra high Millimetre 1-0.1 cm 30 GHz- cloud detection radar.
frequency (EHF) 300 GHz
In the far field on the antenna axis, power density (Pd) can be
calculated from the formula:
Pd = GPt/ 4pi d2 = AeP/lambda2 d2
where G is the far-field gain, Pt is the power delivered to the
antenna (W), d is the distance from the antenna (m), lambda is the
wavelength (m) and Ae is the effective area of the antenna (m2).
G, the far-field gain of the antenna, represents the ratio of the
observed power density, on axis, to the power density from a point
source having the same output power and emitting equally in all
directions.
4.3.2 Near-field exposure
When the distance is not great compared with the antenna
dimensions, the power density tends to vary inversely with d instead
of d2 (as in the far field), and may display interference
patterns. Radiations from different parts of the antenna, having the
same wavelength, combine in various phases. For parabolic antennae,
the maximum power density ( P d) expected in the radiated near
field can be estimated from:
Pd = 4 Pt/ Ae
where Pt is the transmitted power and Ae is the effective
area of the antenna. This expression will generally overestimate the
power density. A fuller discussion of this relationship is provided by
Hansen (1976) and Hankin et al. (1976).
Effects of ground reflections could increase Pd by a factor
of 4 or even more if focusing effects are present. These predicted
values of maximum power density should be within ±3 dB (i.e., within a
factor of the true maxima for most horn antennae and circular
reflector antennae). However, different antenna illumination functions
may produce near-field power densities that may be higher than those
predicted. It should be recognized that the equation is only suitable
for obtaining approximate power densities to use as a rough guide.
More precise values require careful measurements (Bowman, 1970, 1974;
Ruggera, 1977).
In the case of low frequencies or large aperture antennae, the
existence of potentially hazardous reactive near fields becomes
relevant. These electric and magnetic fields are calculable only with
reference to the geometry of the specific antenna and source. For
instance, exact equations for the electric and magnetic fields
generated by a small electric dipole contain terms in
lambda/ d,lambda/ d2, and lambda/ d3. When d is much
smaller than lambda, the lambda/ d3 terms predominate and this is
referred to as the reactive near field. Objects within this region may
couple with the source and extract energy. When lambda/ d approaches
1, all terms contribute and this is sometimes called the intermediate
field. When lambda/ d is substantially less than 1, the conditions
are those of the far field.
4.4 Facilities for Controlled Exposure
Controlled exposure facilities are required for the calibration of
instruments used in measuring power density, for the exposure of
experimental animals in the study of effects, and for the exposure of
phantoms (models) and carcasses in studies on absorbed energy and its
distribution. The unrepeatability of much of the early biological work
has been ascribed to the inadequacy of exposure facilities. Large
gradients in field intensities are very undesirable and preferred
methods make use of either far-field exposures carried out under
conditions in which reflections are reduced to a minimum (e.g., in
anechoic chambers), or using guided wave techniques. The basic premise
is that known exposure conditions can be established by a combination
of measurement and calculation. Except in laboratories with a
responsibility for maintaining primary standards, it is probably
preferable to use the far-field or guided wave methods to obtain
suitable exposure conditions, and to measure the radiation field using
an instrument that has been calibrated at a primary laboratory.
The methods of instrument calibration have been described in
detail by Engen (1973), Baird (1974), and Bassen & Herman (1977) and
are summarized in the following section. The principles apply equally
to animal exposure.
4.4.1 Free space standard field method
There are several variations of this method, but the objective is
always to establish a known calibration field in free space. The most
common experimental arrangement is shown in Fig. 9. As discussed in
section 4.3.1, the power density ( P d) at a point on the
transmitting antenna is given by:
Pd = GP t/4pi d2
where Pt is the power delivered to the transmitting antenna,
G is the effective gain of the transmitting antenna, and d is the
distance from the antenna. The gain is normally determined in advance,
and Pt and d are measured as part of the regular calibration
procedure.
The most convenient method of determining Pt is by sampling
forward or incident and reflected powers. The incident power Pi
and the reflected power Pr are monitored, and Pt is obtained
from the relation Pt = Pi - Pr. The high quality,
broadband equipment available together with methods for its use in
determining P i are described in Bramall (1971), Engen (1971),
Aslan (1972), and Bowman (1976). The methods cited are for calibrating
power meters, but the same techniques can be applied for the
calibration of antennae, if corrections are made for mismatch effects,
including those from animal exposure.
The principal sources of error in the free space method are
multipath interference and uncertainties in the determination of gain.
Multipath effects have often been overlooked, but every calibrating
facility will have some reflections associated with the walls,
equipment, and probe support structure. These reflections may cause
the field in the calibrating region to be significantly different from
that predicted. Even high-quality anechoic chambers are not perfect
and should be evaluated, if the greatest accuracy is desired.
Calibration errors due to multipath effects can be reduced by
observing the probe response as a function of position and averaging
the results. This is sometimes referred to as the multiple position
averaging technique and useful discussions of the method can be found
in Bowman (1974), Bassen & Herman (1977), Swicord & Cheung (1977), and
Swicord et al. (1977).
4.4.2 Guided wave methods
The fields inside a waveguide can be accurately calculated and, in
some cases, are sufficiently uniform to be considered for calibration
purposes (Hudson, 1966; Hudson & Saulsbury, 1971). The main advantage
of such a system is that it requires considerably less power and
space. One disadvantage is that the maximum transverse dimension of a
rectangular waveguide must be less than the free space wavelength of
the highest calibration frequency, in order to avoid higher order
modes which result in complicated field distributions. Thus, the
method is generally used for frequencies below 2.6 GHz, since the
device being calibrated must be small compared with the guide
dimension.
The probe to be calibrated is usually inserted into the wave-guide
through a hole in the side wall and positioned in the centre of the
guide, where the field in most nearly uniform. It is difficult to
estimate the total uncertainty of this method, because the field
intensity will be modified by the size and nature of the probe being
calibrated. A careful error analysis of this problem has not been
completed, but it appears that, if the maximum probe dimension is less
than one third of the smaller waveguide dimension, the total
uncertainty should not exceed ± 1 dB (22%). Woods (1969) described a
system which operated from 400 to 600 MHz with an estimated
uncertainty in the field intensity of ± 0.5 dB (12%). Later results at
2450 MHz have been reported by Aslan (1972) with claims of higher
accuracy.
Other types of guided wave structures can be used reliably to
establish uniform fields for calibration purposes in the RF frequency
range below about 500 MHz, where free space techniques become
difficult and standard waveguides are unavailable or inconvenient. The
two most commonly used structures are the parallel plane line and the
Transverse Electromagnetic Mode (TEM) cell (Crawford, 1974). Both
structures can be used to produce transverse electromagnetic waves
with the same wave impedance (377OMEGA) as a plane wave in free space,
a feature which makes them desirable for calibration purposes.
Furthermore, the fields can be calculated with sufficient accuracy for
many calibration purposes.
A basic TEM cell is illustrated in Fig. 10. The principal
advantage of this structure over the parallel plane line is that the
TEM cell is fully shielded, thus eliminating extraneous radiation that
may interfere with electronic equipment. The basic unit is a section
of two conductor transmission lines. As shown in the figure, the main
body of the cell consists of a rectangular outer conductor and a flat
centre conductor located midway between the top and bottom walls. The
field intensity in the centre of the cell can be quite uniform, and
the wave impedance throughout the cell is very close to the free space
wave impedance. It is mainly because of these features that this type
of cell is used for calibrations.
The dimensions of such a cell are adjusted according to the
desired upper frequency limit.
4.4.3 Standard probe method
A stable and reliable probe, which has been accurately calibrated
by one of the previously described techniques, a "transfer standard",
is used to measure the field intensity over a particular region in
space (or in a guided system) produced by an arbitrary transmitting
antenna. The probe to be calibrated is then placed in the same
location in the field and the meter reading compared with the known
value of the field. The only requirements are that the transmitter
should generate a field which has the desired magnitude, is constant
in time, and is sufficiently uniform over the calibrating region.
Accuracies of about ± 0.5 dB (12%) should be attainable. This method
is the simplest, and may ultimately prove to be the best method of
calibrating hazard meters for general field use Baird (1974). The
advantages of this method are its convenience, reliability, and
simplicity. Potential sources of error, when using the transfer
standard to calibrate another probe, are the possible differences in
the receiving patterns of the two probes, especially in the near
fields of a source of radiation, and the errors due to scattering from
probes under test.
5. MEASURING INSTRUMENTS5.1 General Principles
Most power density instruments are composed of 3 basic parts: the
sensor, connecting leads, and meter unit. This configuration reduces
perturbation of the field in the immediate vicinity of the sensor to a
minimum and, in surveys of potentially hazardous equipment, may help
in reducing the exposure of the operator. Neither leads nor meter unit
should respond to the radiation being measured or serious error can
ensue. The instrument should respond only to microwave and RF fields
and not, for instance, to light and infrared radiation, or to static
and low-frequency electric and magnetic fields. Comparatively few
instruments are likely to meet these requirements in full.
The basic principles of instrument calibration with the
uncertainties associated with the different methods have already been
discussed in section 4.4. The same accuracy cannot be expected or
achieved when using the meters for making practical measurements in
surveys because: (a) hazard meters are usually calibrated in nominally
plane-wave fields, which are seldom encountered in practice, and the
sensor may not respond in the same way to non-planar fields; and (b)
in most calibration methods, only the sensor (probe) is exposed to the
field, while, in practice, the complete system, including the
indicating unit and connecting cable, is immersed in the field. Even
when these do not respond to the radiation, the radiation fields will
be perturbed by their presence and that of the operator. If good
measurement procedures are followed, accuracies of 2 dB can be
achieved.
5.2 Types of Instruments in Common Use5.2.1 Diode rectifier
In these instruments, small antennae terminate in single or
multiple diodes. Multiple diodes and antenna elements arranged in
suitable configurations can be used to sum all electric field
components enabling measurements to be made, irrespective of
polarization and direction of incidence. Three orthogonal elements are
necessary and sufficient for such an isotropic instrument.
Some units, now in use, employ a single diode combined with a
short dipole or small loop antenna. The sensitivity of these
instruments changes with their orientation, with respect to the plane
of polarization of the E or H field. They must, therefore, be
oriented so that the maximum value can be read -- a process that can
be tedious and time-consuming. Such instruments are, however, useful
for identifying and measuring individual field components.
An orthogonal dipole array or multiple diodes and dipoles arranged
in a single plane will respond well to all signals polarized in the
plane of the array, but not to components polarized at wide angles to
the array. Such units must also be oriented to obtain the maximum
field readings.
All these instruments are basically power density sensitive in the
far field, that is, at low levels, the rectified voltages are
proportional to the square of the E field (i.e., to the power
density). When adapted to broadband operation, the upper frequency
range is, at present, about 18 GHz. The corresponding low frequency
limit is about 0.5 MHz.
Diode characteristics depend directly on ambient temperature and
variations in output with ambient temperature may be in the order of
tenths of a dB (several percent) per degree Celsius. Diode units may
also be modulation sensitive, with errors in reading dependent on the
form of modulation.
5.2.2 Bolometer
In bolometric instruments, the microwave/RF currents cause heating
and induce a change in some physical property, most commonly the
resistance of a thermistor. A measure of the power density would then
be the resulting imbalance of a bridge circuit containing the
thermistor. For small deviations from balance, the bridge output is
proportional to the temperature of the thermistor and therefore to the
square of the electric field, i.e., to the RF power dissipated in the
thermistor. The thermistors used have a positive temperature
coefficient. Thus, this type of instrument can withstand large
overloads without damage. As the power density increases, the
resistance of the element increases, causing a mismatch condition and
the power absorbed by the thermistor is also inversely proportional to
its resistance. Both effects limit the power absorbed by the element.
These units may exhibit drift in the zero reading and loss in
sensitivity caused by changes in ambient temperatures.
5.2.3 Thermocouple
The detection elements in thermocouple-type radiation monitors are
generally thin-film type thermocouples. The films perform the
simultaneous functions of lossy antenna element and temperature
detector. The output from the thermocouple is proportional to the
square of the electric field and the units are relatively independent
of ambient temperature (Bassen et al., 1977). Hot and cold junctions
of the thermocouple are in extremely close proximity and very stable.
Variation in sensitivity is of the order of 0.1% per°C. The use of
small, thin, resistive films provides very broad bandwidth. More
detailed discussions of these, and other types of instruments, can be
found in Aslan (1972), Bowman (1976), Eggert & Goltz (1976), and
Eggert et al. (1979).
6. MICROWAVE AND RF ENERGY ABSORPTION IN BIOLOGICAL SYSTEMS
Electric and magnetic fields are induced within a biological
system exposed to microwave or RF energy. To understand the resulting
biological effects, it is necessary to determine the induced field
strength at various internal points of the system. Knowing the
electrical and geometrical characteristics of the irradiated object
and the external exposure conditions, it is possible, in principle, to
calculate the rate at which energy is absorbed throughout the interior
of the irradiated object.
The magnitude of interior and exterior scattered and reflected
fields depends on many factors: the frequency and configuration of the
incident field; the electrical properties of the various layers
(tissues) of which the irradiated system is composed; the shape, the
size relative to wavelength, and the relative orientation of the
system. Biological systems are usually of complex exterior and
interior geometry, and consist of several layers with various
electrical properties (complex permittivity). As a result, the
internal energy deposition in biological systems will be nonuniform.
Depending on the thermal properties and blood flow of tissues, there
can be marked differences in the magnitude and rate of increase in
temperature, and thermal gradients can result. A review on the
interaction of microwave and RF radiation with living systems has
recently been completed by Stuchley (1979).
6.1 Methods of Computation
Methods of computation for predicting internal energy deposition
using various approximate mathematical models of human and animal
bodies have been developed. These show reasonable agreement with
experimental measurements of energy absorption in phantom models and
animal carcasses (Guy, 1971, 1974; Johnson & Guy, 1972).
Theoretical analyses have led to the prediction of the resonant
absorption of energy in both the whole and parts of the body of human
models and animals. The effects of such variables as frequency and
polarization of the field, the size and shape of the exposed body, and
the surrounding environment, ground plane, and other objects have been
evaluated.
Details concerning computational and experimental techniques, data
on specific absorption rates within the range of 10 kHz-100 GHz in man
and laboratory animals, as well as pertinent reference lists, can be
found in the three editions of the "Radiofrequency radiation dosimetry
handbook" (Johnson et al., 1976; Durney et al., 1978, 1980). In the
most recent edition of the Handbook, several models relevant to
exposures in the near-field of the radiation source have been
included.
The intensity of the internal electric field or the amount of
energy absorbed per unit time per unit mass (the specific absorption
rate (SAR)) are both used in radio-frequency and microwave dosimetry.
Most frequently used units of SAR are W/kg and mW/g. Further
discussion on SAR follows in section 6.3.
6.2 Experimental Methods
Measurements of internal electric fields within dielectric media
are possible if a small, insulated dipole array is used. Such a device
has been developed in miniature form and used to measure internal
microwave fields in phantoms and living animals with uncertainties of
less, than 1 dB. The advantage of the implantable electric field probe
method over thermal dosimetric techniques is the greater sensitivity
of the field probe, allowing the use of microwave and RF sources with
much lower power outputs. Thus, the energy deposition can be mapped in
a biological body or a scan through a phantom exposed to only moderate
levels of microwave or RF energy.
Thermal measurements in phantoms or animal carcasses can yield
direct data on SAR. Small thermistor probes with non-perturbing
resistive leads, and optical fibre probes with temperature-sensitive
sensors using a light source have been developed (Aslan, 1972; Cetas,
1975; Livingston et al., 1975; Bowman 1976; Deficis & Prou, 1976;
Bassen et al., 1977). Thermographic cameras used in conjunction with
sectioned phantom models or carcasses can record the heat distribution
in an entire plane. High intensity exposure fields have to be employed
to yield significant increases in temperature.
6.3 Energy Absorption
Biological systems are lossy dielectrics characterized by limited
conductivity. The losses originate from the movement of free ions
(conduction loss) and molecular rotation (dielectric loss). Thus,
electromagnetic waves, propagating through a biological medium,
interact with it, and energy transfer occurs. This results in
attenuation of the field and an increase in the kinetic energy of the
molecules of the medium, i.e., in heating. The degree of attenuation
of the field depends on the dielectric properties of the medium, and
these change with the frequency of the incident field. The real and
imaginary parts of the complex permittivity generally decrease with
increasing frequency.
The above statements present, in a simplified form, the classical
theory of microwave and RF energy absorption, which was developed by
Schwan and his school (Schwan & Piersol, 1954, 1955; Schwan, 1971,
1976). The latest restatement of this approach (Schwan, 1978) may be
summarized as follows: "Among the established effects in biological
systems the most important is heat development but direct field
interactions with membranes, biopolymers, and biological fluids are
all possible". All energy deposition, however, takes place because of
conduction losses, molecular movements, and biopolymer rotation.
During the last few years, the concept of the specific absorption
rate (SAR) has been developed for quantifying microwave and RF
effects.
As mentioned earlier, the specific absorption rate is defined as
the rate of energy absorption per unit mass of an exposed object. For
steady-state sinusoidal fields, the SAR is directly proportional to
the tissue conductivity, the square of the electric field, and
inversely proportional to the mass density. The relationship is more
complex in pulsed or modulated fields, if the intrinsic properties of
the medium are non-linear. However, since the SAR is related to the
intensity of the internal electric field, this concept can be used
independently of the nature of the interaction mechanism responsible
for biological effects. This stems from the fact that it is the
internal electric field intensity that quantitatively describes the
interaction. Nevertheless, it may not be the only factor, e.g.,
frequency and/or modulation of the radiation field may strongly affect
biological effects. Consequently, the nature of the radiation fields
should always be considered in addition to the SAR.
The SAR is a measure of the absorbed energy which may or may not
all be dissipated as heat. The temperature is a function of the SAR,
but it is also a function of the thermal characteristics of the
absorber (i.e., the size, shape, thermal conductivity).
The values of the SAR averaged over the whole body and the
distribution of the SAR have been estimated theoretically and measured
experimentally in models and experimental animals for various exposure
conditions. For human subjects, the average SAR for exposures in the
far field may reach a peak in the frequency range of 30-200 MHz,
depending on various factors associated with the specific exposure
situation (Johnson et al., 1976; Durney et al., 1978, 1980). Fig. 11
presents the average SAR in man and experimental animal models at an
incident power density of 1 mW/cm2 in free space (far-field)
conditions. The graphs in Fig. 11 (page 46) show the importance of
size, frequency, and orientation, while Fig. 12 shows values of
average SAR at the resonant frequency for several exposure conditions
for models of man and 2 sizes of rats. This mathematical modelling is
only possible for greatly simplified models.
In addition to the average SAR, the SAR distribution in many
models has been calculated. Much of this work can be found in reports
by Shapiro et al. (1971), Lin (1976), Gandhi et al. (1979), Kritikos &
Schwan (1979), and is summarized in the latest edition of the
Dosimetry Handbook (Durney et al., 1980).
In the absence of adequate knowledge concerning the mechanisms of
interactions between microwave energy and biological systems, and in
the light of the limitations inherent in the SAR, the following
conclusions can be drawn:
(a) SAR alone cannot be used for the extrapolation of effects from
one biological system to another, or for the extrapolation of
biological effects from one frequency to another.
(b) Curves for exposure which produce equivalent SAR for a given
body over the microwave/RF energy spectrum may be used to predict
equivalent average heating, provided data concerning heat dissipation
indicates equivalent heat dissipation dynamics. Such curves cannot,
however, be used as the only basis for predicting biological effects
or health risks over the microwave/RF spectrum, since from current
knowledge, it is not possible to state that equivalent average energy
absorption rate for given radiation frequencies is associated with
equivalent biological effects.
6.4 Molecular Absorption
Despite the photon energies, some recent theoretical explanations
of experimental observations strongly indicate the possibility of
interactions at the molecular level. Proton tunnelling, changes in the
conformation of molecules, and cooperative mechanisms have been
envisaged (Fröhlich, 1968; Illinger, 1971, 1974; Cleary, 1973, 1978;
Rabinowitz, 1973; Grodsky, 1975; Keilmann, 1977).
It has been postulated (Fröhlich, 1968; Rabinowitz, 1973) that
microwaves in the frequency region of 60-120 GHz may influence
macromolecules in biological systems, altering such functions as cell
division, and virus inactivation or activation. Effects on enzyme
systems, DNA-protein structures (chromosomes), and cell membranes are
possible (Grundler & Keilmann, 1978; Pilla, 1979; USSR Academy of
Sciences, 1973). Physical experimental techniques need developing and
further studies on biological effects are necessary. Similar
mechanisms may be operative at lower frequency ranges (Kaczmarek &
Adey, 1974; Adey, 1975; Grodsky, 1975; Bawin & Adey, 1976) and the
present status of knowledge about the molecular absorption of
microwaves and RF in biological systems has been summarized by Straub
(1978) who states:
"Absorption of non-ionizing electromagnetic (EM) radiation by
biologically important molecules can occur by many different
mechanisms over the frequency range from several hertz through the
millimeter microwave region. The absorption of EM radiation is
determined by the bulk dielectric properties of living tissues,
cells and biomolecules in solution. However, the existence of
diverse and complex molecular structures characteristic of
biological systems makes it necessary to consider the details of
absorption and dissipation of EM energy. In addition, the
biological function of the molecular species absorbing energy
needs to be studied to understand the significance of the EM
absorption. Among many possible examples the following five are
given: (1) The network of membranous lipid-containing structures
within and at the outside limit of cells poses a series of
barriers to thermalization of the absorbed radiation. Thus,
adiabatic conditions may be maintained in small membrane bound
volumes for much longer periods of time than in simple solution.
Large thermal gradients and temperature elevations can result. (2)
Subsequent temperature elevation may cause membrane structures or
complex protein assemblies to pass through phase transitions,
altering their properties. (3) Spatial anisotrophy in the
arrangement of large molecular assemblies, as found in
mitochondria and ribosomes, results in specialised functions which
can he completely changed if some of the molecules are rotated or
translated by EM radiation. (4) Quantum effects such as proton
tunnelling with resulting isomerization of DNA base pairs may also
be influenced by EM radiation. (5) Otherwise random motion of
"gates" in excitable channels of nerve membranes may be brought
into forced oscillation by EM radiation, with resultant membrane
depolarization. Detailed knowledge of structure and function of
the biological system thus reveals many perturbations which might
be induced by EM absorption, and, conversely, EM radiation can be
used to probe biological structures and function."
7. BIOLOGICAL EFFECTS IN EXPERIMENTAL ANIMALS
During the past thirty years, research has been devoted to various
aspects of the interactions between microwave and radiofrequency
radiation and biological materials. Unfortunately, most experiments
have tended to report biological effects as phenomena rather than
attempting to establish whether such radiation presents a health risk
to man and other biota. In Czechoslovakia, Poland, and the USSR, a
continuous research effort made over the last 25-30 years has resulted
in numerous research reports and reviews (Presman, 1968; Marha et al.,
1971; Baranski & Czerski, 1976). In the past 20-25 years, interest in
this field of studies has increased in the USA, first with the
establishment of the Tri-service Programme in 1956 and then other
programmes in later years.
It is impossible to review the numerous studies (see
bibliographies by Glazer et al., 1976; Glazer & Brown, 1976; Glazer et
al., 1977) related to the biological effects of microwave radiation
and only those most pertinent to the evaluation of potential
biological hazards have been cited. Potentially beneficial effects of
microwave radiation are outside the scope of this document.
Only limited information is available from studies of human
subjects directly exposed occupationally or experimentally to
microwave radiation. Most of the data on possible harmful effects are
based on studies of separate cells, simple organisms, animals, and
models, making it difficult to extrapolate such experimental results
to man.
Radiant energy absorption in the living system followed by direct
interaction with biophysical or biochemical processes, may be defined
as the primary interaction. Changes in the structure and function of a
biological system as a result of the primary interaction are
considered to be biological effects. Immediate biological effects
arising at the site of the primary interaction may induce further
indirect changes, both acute and chronic.
The analysis of data on effects requires the consideration of a
sequence of events: the physical interaction followed by physiological
reactions -- local and generalized, and immediate and delayed
biological effects. In addition, frequent activation of adaptive
mechanisms may lead to their exhaustion via the classical sequence of
events of stress-adaptation-fatigue. Consequently, the effects of
single and repeated exposures should be considered separately, even
when exposures take place under identical conditions.
For many years, the primary interaction of microwave and RF
radiation with living systems was considered almost exclusively in
terms of electromagnetic field theory (Schwan, 1976, 1978). It was
concluded that the conversion of the absorbed energy into kinetic
energy of molecules (i.e., heat) was the only significant mechanism
involved. However, there are discrepancies between some empirical
observations and the theoretical explanations available (Cleary, 1973;
Baranski & Czerski, 1976; Dodge & Glaser, 1977), which indicate that
"non-thermal" effects may play some role. Direct interference with
bioelectric phenomena (as seen on the electroencephalogram and the
electromyogram) and the role of electromagnetic fields in the
transmission of biological information was suggested by Presman
(1968), but these hypotheses need experimental verification.
Interaction of microwave energy at the molecular level has been
postulated to explain the primary interaction between microwaves and
parts of living systems such as membranes (Fröhlich, 1968; Adey, 1975;
Bawin et al., 1975; Grodsky, 1975; Bawin & Adey, 1976; Grundler &
Keilman, 1978; Pilla, 1979).
7.1 Hyperthermia and Gross Thermal Effects
Numerous biological and pathophysiological effects have been
attributed to temperature increases in the tissue resulting from
absorption of microwave energy. Thermal effects leading to gross
injury or death have been studied in a number of experimental animals
and are described here. More subtle effects of thermal origin, caused
by absorption of microwave energy will be described in later sections.
The absorption of microwave energy often results in an increase in
temperature. If the rate of increase exceeds the ability of the
thermoregulatory system of the organism to dissipate heat,
hyperthermia will occur, followed by injuries such as burns,
haemorrhage, tissue necrosis (Cleary, 1978), and death. The extent of
the damage depends on the thermal sensitivity of the tissue. With
partial body exposure, highly vascularized tissues show greater
resistance to thermal damage, because of the more efficient heat
dissipation. Microwave-induced death in an experimental animal depends
not only on the quantity of absorbed energy but also on the rate of
absorption, the animals thermoregulatory system, its physiological
status, and the environment. Quite different responses to microwave
exposure have been observed in various species. Table 10 gives the
survival times of various experimental animals following prolonged
continuous exposure to microwaves at different frequencies.
Dogs exposed to microwaves at frequencies of 2.86 GHz, 1.28 GHz,
and 200 MHz (Michaelson, 1971, 1973) and a power density of
165 mW/cm2 experienced three distinct phases of hyperthermia. First,
the body temperature increased by 1-1.4°C after about 30 min (the
extent of the delay depending on the exposure frequency). Second, at
thermal equilibrium which lasted about 1 h (longer at lower
frequencies), the rectal temperature stabilized at between 40.5°C and
41°C. Finally, the thermoregulatory system could not dissipate the
heat rapidly enough, the rectal temperature quickly rose above 41°C,
and the animal succumbed. Similar thermal responses have been
Table 10. Power densities and exposure times until thermal death in a number of animal
species at various frequenciesa
Power Exposure Rectal
Species density time Frequency temperature Reference
mW/cm2 min MHz increase
°C
dog 350 15 200 5 Addington et al. (1958)
330 15 200 4 Michaelson (1971)
220 21 200 4 Addington et el. (1958)
165 270 22 800 4-6 Ely & Goldman (1956)
rabbit 300 25 2 800 6-7.5 Ely & Goldman (1956)
165 40 2 800 &gt 4 Michaelson (1971)
165 30 200 6-7 Ely et al. (1954)
100 103 3 000 4-5 Gordon (1966)
rat 400 13-14 10 000 7 Gordon (1966)
300 15 3 000 8-10 Gordon (1966)
300 15 24 000 5 Ely & Goldman (1956)
150 15 2 400 - Michaelson (1971)
100 25 3 000 6-7 Gordon (1966)
100 5-120 mm--dm - Gordon (1966)
80 35 2 400 - Michaelson (1971)
80 56 24 000 - Deichmann (1966)
50 80 24 000 - Deichmann (1966)
40 90 3 000 7 Gordon (1966)
40 30-180 mm--dm Gordon (1966)
30 135 24 000 - Deichmann (1966)
10 &gt 5 h mm--dm - Gordon (1966)
mouse 180 3 24 000 Deichmann (1966)
150 5 24 000 Michaelson (1971)
80 13 24 000 Deichmann (1966)
80 13 24 000 Michaelson (1971)
50 35 24 000 Deichmann (1966)
50 35 24 000 Michaelson (1971)
30 140 24 000 Deichmann (1966)
a "Adapted from: Baranski & Czerski (1976).
described for dogs with body weights between 4 and 20 kg. No period of
thermal equilibrium was observed in rats and rabbits exposed at the
same power density (165 mW/cm2) (Michaelson, 1973).
Table 11 gives the survival time of rats exposed intermittently to
24 000 MHz microwaves at 300 mW/cm2. These data provide information
on a situation corresponding to exposure to a rotating antenna. This
type of intermittent exposure prolongs the survival time of the
irradiated animals.
Table 11. Survival time of rats following intermittent exposure
to 24 000 MHz microwaves at 300 mW/cm2 depending on
the relationship between duration of exposure period
and duration of exposures on-offa
Operation period of Survival time equal to effective
the transmitter irradiation time
(s) (min)
on off
60 60 16.5
5 15 28
3 3 40
30 60 39
10 20 65
3 6 95
60 180 28
10 30 76
3 9 110 to 120
30 120 70 to 75
15 60 Over 100
a From: Baranski & Czerski (1976) based on Deichmann et al.
(1959).
Table 12 provides a summary of data (Baranski & Czerski, 1976) on
the mass, body surface, and basal metabolic rate of commonly used
experimental animals. These data can be used to compare the
experimental results of a microwave-induced thermal load with the
animal's ability to dissipate heat (its thermoregulatory system).
Table 12. Mass, body surface and metabolic rate in various experimental animalsa
Man Dog Rabbit Monkey Guineapig Rat Mouse
Mass (kg) 65 15.0 3.5 3.2 0.8 0.2 0.02
Body surface (m2) 1.83 0.85 0.2 0.26 0.071 0.081 0.085
Basal metabolic
rate (W/m2) 45.5 46.0 40.5 30.5 33.7 45.2 26.2
a From: Baranski & Czerski (1976).
In the results presented in Table 10, it was generally assumed
that the area of the species exposed was approximately one third of
the body surface, the incident energy was totally absorbed, the heat
dissipation index was 12 W/m2/°C, and that the initial temperature
difference between body surface and surrounding air was 10°C.
Environmental conditions can influence the thermal response
(Baranski et al., 1963; Michaelson, 1971). At an ambient temperature
above normal (40.5°C), the animal's thermoregulatory system can
maintain a normal body temperature, but is not able to cope with an
additional thermal load produced by microwave exposure. However, at a
lower ambient temperature (11°C), after an initial period of
adaptation, the microwave radiation does not significantly affect the
animal's rectal temperature (Michaelson, 1973).
The influence of environmental conditions on hyperthermia induced
by microwave radiation exposure can be summarized as follows: (a)
increasing ambient temperatures and humidity enhance thermal stress;
and (b) increased air velocity decreases thermal stress.
In a study by McLees & Finch (1973) in which rats were exposed to
24 GHz and 300 mW/cm2, it was shown that body cover also affected
hyperthermia. Animals with and without hair died within 15.5 and 18.5
minutes, respectively, indicating that clothing could be expected to
enhance the thermal effects of radiation, unless such clothing
shielded from, or reflected microwave energy.
When dogs were anaesthetized using sodium pentobarbital,
chlorpromazine, or morphine, impaired thermoregulatory responses and
increased susceptibility to radiation thermal stress were observed
(McLees & Finch, 1973; Baranski & Czerski, 1976).
Repeated exposure resulted in physiological adaptation via the
classical sequence of stress-adaptation-fatigue. Daily exposure of
dogs to 1280 MHz microwaves for 6 h per day, 5 days per week, for one
month at a power density of 100 mW/cm2, resulted in an increase in
rectal temperature with each exposure during the first week. During
the following 3 weeks, temperature increases were moderate, and a
progressive reduction in the pre-exposure temperature was observed as
the number of exposures increased (Michaelson, 1973). These results
have been confirmed for other species (Gordon, 1966; Phillips et al.,
1973).
The blood circulation was considered to be an effective system for
distribution of the heat generated throughout the body (Michaelson,
1971), and until recently, the thermal effects of microwaves in
animals were mainly considered in terms of "volume heating". Guy and
his associates (Guy, 1971, 1974; Johnson & Guy, 1972) using phantom
models, developed elegant thermographic techniques and demonstrated
convincingly very nonuniform deposition of microwave energy, expected
to result in nonuniform deep body heating. In physiological terms,
this means that absorbed energy may cause local thermal stimulation or
gross effects on different organs depending on the exposure level.
7.2 Effects on the Eye
Studies on the effects of microwave radiation on the eye were
carried out as early as 1948 (Richardson et al., 1978). Most animal
studies have been conducted on the New Zealand white rabbit because
its eye is similar to the human eye.
Investigations to determine cataractogenic radiation levels and
lengths of exposure at various frequencies have been conducted in both
the far and near fields. In far-field studies, the whole of the body
of the animal is exposed but, in some cases, this results in the
animal's death. Near-field techniques involve exposing the eye at some
distance from the source and permitting air to circulate against the
eye, or exposing the eye by direct contact with a source of microwave
energy, so that there is no air circulation. The conditions of
exposure have a considerable influence not only on the development of
cataracts but also on their location in the eye. When air circulation
is permitted, the exposure causes opacities to develop in the
posterior subcapsular cortex of the lens. Without an air gap,
opacities develop in the anterior subcapsular cortex (Carpenter et
al., 1974b).
Guy and his colleagues (1975b) have recently determined threshold
power density levels and durations of exposure for cataract formation
in rabbit eyes with a single exposure to 2.45 GHz near-field
radiation. Their results are in good agreement with earlier data
obtained by Carpenter and his co-workers (1974b) as shown in Fig. 13.
At 2.45 GHz, the maximum temperatures occurred near the posterior
surface of the lens, and irreversible changes in the lens took place
in the posterior cortical area only. Other changes in the exposed eye
were found to be transient in nature and disappeared within two days
of irradiation. The minimum power density level at which cataracts
were formed appeared to be 150 mW/cm2 for 100 min corresponding to a
maximum specific absorption rate in the vitreous body of 138 W/kg. The
threshold temperature in the eye for cataract formation was estimated
to be about 41°C (Guy et al., 1975b).
To investigate the mechanism by which microwaves produce cataracts
at 2.45 GHz, rabbits were subjected to general hyperthermia and local
heating of the lens (Kramer et al., 1976). Rabbits, under general
hyperthermia, were kept at a temperature above 43°C for 35 minutes.
After 4-6 months, the only cataracts observed occurred in eyes damaged
by insertion of the temperature probes. The authors concluded that
basic differences occur when heating by means of microwave energy and
by convective hyperthermia. Eyes irradiated with microwaves show a
characteristic temperature gradient, with the highest temperature
behind the lens, whereas in hot bath experiments, the highest
temperature occurs at the surface of the cornea. Further high-level
microwave exposure raises the eye temperature within minutes, compared
with at least 2 h in the hot water bath. Thus, a sharp temperature
gradient and a high rate of heating rather than gradual, more uniform
heating may be necessary to produce cataracts (Kramer et al., 1976).
In studies to investigate the relative cataractogenic effects of
exposure to two frequencies, 2.45 and 10 GHz, a special dielectric
lens was used to irradiate the eyes of New Zealand white rabbits,
selectively. With a constant power density, exposure to 10 GHz induced
a higher intraocular temperature than exposure to 2.45 GHz. However,
when the animals were exposed to these frequencies for the same length
of time, cataracts were induced at lower power densities at 2.45 GHz
than at 10 GHz (Table 13). Although opacities formed in the posterior
subcapsular cortex of the lens at both frequencies, their initial
appearance and subsequent development differed. Radiation at 2.45 GHz
induced posterior cortical banding within 1 or 2 days, followed by the
appearance of small granules along or on the horizontal line of the
posterior suture. Occasionally small vesicles developed. Some
opacities also had a fibrillar, cotton-like appearance, and
superficial damage, such as pupillary constriction and hyperaemia of
the bulbar and palpebral conjunctiva was observed within 24 hours
(Hagan & Carpenter, 1976).
In one of the very few investigations of chronic, low-level
exposure of rabbits' eyes (2 mW/cm2 for 8 h per day, 5 days a week,
for 8-17 weeks at 2.45 GHz), ocular changes were not observed up to 3
months after termination of exposure (Ferri & Hagan, 1976).
Table 13. Production of cataracts in the eyes of rabbits by a single 30-min exposure
to 2.45 GHz or 10 GHz
2.45 GHz 10 GHz
Incident power Number of Development of Number of Development of
density (mW/cm2) experiments lens opacities (%) experiments lens opacities (%)
275 12 8 -- --
295 12 67 -- --
310 12 58 12 0
325 12 100 -- --
345 2 100 12 50
375 -- -- 12 67
410 -- -- 11 82
440 -- -- 2 100
a From: Hagan & Carpenter (1976).
When the cataractogenic power density levels for continuous wave
and pulsed radiation were compared at a few frequencies, no
differences in the threshold levels for cataractogenesis were found
(Carpenter & Van Ummersen (1968); Carpenter (1969); Birenbaum et al.
(1969); Williams & Finch (1974); Weiter et al. (1975). The average
power density, not the peak power density, appears to be the critical
field parameter in cataract induction.
Most authors including Belova (1960), Carpenter et al. (1974b),
Paulson (1976), Kramer et al. (1978) and Steward-Dehaan et al. (1979)
have tended to relate microwave cataracts to the secondary effects of
local temperature increase. The conventional view is that, as the
crystalline lens does not have its own blood supply, it is easily
overheated with consequent damage to capsular cells and denaturation
of the protein in the lens.
Studies have been performed to determine if cataracts can be
formed by an accumulation of exposures at subthreshold levels. In one
experiment, rabbits' eyes were exposed for 3 min to 2.45 GHz radiation
at a power density of 280 mW/cm2 (5 min exposure was required to
induce a cataract after a single exposure). When the 3-min exposure
was given once a day for 5 days, the animals developed cataracts.
However, if the eyes were exposed under the same conditions, but with
a break of 7 days between exposures, cataracts did not develop
(Carpenter, 1969). In an earlier study, rabbits' eyes were exposed to
2.45 GHz at a power density of 80 mW/cm2 for 60 min daily, for 10 or
15 days (Carpenter & Van Ummersen, 1968). Cataracts appeared 1-6 days
after treatment. However, the authors later indicated that the power
density measurement was inaccurate and that subsequent measurements
showed that the actual power density was greater than 80 mW/cm2.
Paulsson et al. (1979) studied the eyes of rabbits exposed to
3.1 GHz pulsed (pulse length 1.4 µs, repetition frequency 300 Hz)
radiation at an average intensity of 55 mW/cm2 (1.3 MW/m2 peak)
either to single exposures of 1-1´ h or, after a series of repeated
1-h exposures, for up to 53 h during 100 days. Degenerative changes in
the retinal neurons and synaptic boutons, and reactive changes in
glial cells were observed only following the repeated exposures. No
evidence was found of increased permeability of the blood-retina
barrier.
Effects of millimetre waves (35 and 107 GHz) at power densities
ranging from 5 to 60 mW/cm2 for 15 min-1 h were investigated in
rabbit eyes by Rosenthal et al. (1976). Corneal damage and epithelial
and stromal injury were observed. Stromal injury appeared at lower
power densities (5 mW/cm2) at a frequency of 107 GHz than at 35 GHz,
but it was concluded that keratitis (inflammation of the cornea) was a
useful criterion for ocular response to millimetre radiation.
Keratitis occurred at lower power densities than those required to
produce other ocular effects such as iritis or lenticular injury. The
recovery rate from stromal injury depended on the frequency of the
radiation and was faster after exposure to 107 GHz.
The following conclusions on the effects of microwave radiation on
the eye can be drawn from these and other data from literature
reviews:
(a) Above 500 MHz, opacities of the eye may be produced when power
densities exceed 150 mW/cm2, if the duration of exposure is
sufficiently long;
(b) Although ocular injury has not been reported at frequencies
below 500 MHz, its possibility cannot be excluded;
(c) The frequency of the microwave radiation influences the type
and location of the injury to the eye;
(d) Exposure conditions, namely whether in the near field or far
field, whole body or selective exposure of the eye, eye exposure with
or without an air gap (to provide cooling), and the temperature of the
animal's body, all influence the power density and duration of
exposure needed to produce eye injury.
(e) Injury to the eye from microwaves appears to be predominantly
thermal in nature, temperature gradients within the eye and the rate
of heating being two major factors in the stress that leads to injury.
Non-thermal effects cannot be excluded but they alone do not appear to
be sufficient to produce effects in the eye, although they may provide
a necessary mechanism of interaction.
(f) As can be seen in Fig. 13 (p. 55) the threshold curve of power
densities versus time to produce eye cataracts is not linear. Exposure
of the eye at each frequency seems to require a threshold microwave
power density below which even continuous exposure does not produce
eye injury. This would appear to exclude the possibility of
cataractogenesis caused by low level chronic exposure, and this was
confirmed in a recent experiment (Ferri & Hagan, 1976).
(g) Pulsed and continuous wave radiation with the same average
power density level seem to possess the same potential for cataract
induction. However, effects from pulsed radiation with a small duty
factor and high peak power cannot yet be excluded;
(h) Cataracts can be produced by repeated exposures to
sub-threshold power density levels. For this cumulative effect to
occur, the levels have to be sufficiently high that a slight but
persistent injury is not fully repaired before another exposure takes
place. However, if the time between exposures is sufficiently long for
repair to take place, cumulative damage is not observed.
7.3 Neuroendocrine Effects
Interaction between the endocrine and nervous systems is very
important to the functioning of the human body. The hypothalamus
within the brain is a control centre involved in the regulation of the
autonomic nervous system, including such visceral functions as
temperature control within the whole body. This gland, coordinated by
the central nervous system (CNS), releases specific factors into the
pituitary portal system, which regulate hormones released by the
endocrine organs. The endocrine system can be considered as a feedback
control system where the hypothalamus, via the pituitary, causes
hormones to be secreted by endocrine glands. Once the endocrine
hormones have reached a certain level, this information is fed back to
the pituitary and hypothalamus, causing a reduction or cessation in
hormone secretion. The system's actions are modified by direct neural
inputs from higher brain centres and peripheral nerves.
Descriptions of the biochemical and neuroendocrine aspects of
exposure to microwaves can be found in recent reviews by Michaelson et
al. (1975) and Cleary (1977).
Dogs exposed to 3 GHz microwaves at 10 mW/cm2 showed a
substantial increase (100%-150%) in corticosteroid levels, a decrease
in blood potassium, and an increase in blood sodium content (Petrov &
Syngajevskaja, 1970). The increase in the corticosteroid levels during
and after irradiation may have been an adaptive reaction, since in
some animals the adrenocortical function becomes inhibited and
sensitivity to microwave radiation increases because of insufficient
release of adrenocorticotropic hormone (ACTH).
Dumanskij & Sandala (1974) found that chronic low-level exposure
of rats and rabbits to 3 cm, 12 cm, and 6 m microwaves at 10 µW/cm2
and below, for 8-12 h per day, for 120 days, reduced cholinesterase
and increased 17-ketosteroid levels in the urine during the 60 days
following irradiation. A reduced amount of ascorbic acid in the
adrenal glands and reduced adrenal gland weight were also observed.
Syngajevskaja et al. (1962) exposed dogs and rabbits (162 animals) to
decimeter waves at 70 mW/cm2 for 30 min and reported increases in
the ascorbic acid concentration in the adrenals, while exposure at
5 mW/cm2 for 30 min caused it to decrease. Changes in glucose levels
in the blood and variations in liver glycogen content were observed;
lactic acid levels were also affected. It has been suggested that a
whole body rise in temperature caused by microwave exposure suppresses
the hormone-producing functions of the anterior pituitary and
adrenals, while exposures not resulting in an increased rectal
temperature enhance hormone production (Petrov & Syngajevskaja, 1970).
No significant alterations were observed in growth hormone or
thyroxine levels in barbiturate-anaesthetized dogs, cranially exposed
to 2.45 GHz microwaves at various power densities (20-80 mW/cm2) for
1 h (Michaelson et al., 1975). When rats were exposed (whole body) for
1 h to 2.45 GHz microwaves at 9 mW/cm2 an increase in growth
hormones was observed, but at 36 mW/cm2 exposure, a significant
decrease was noted (Syngajevskaja et al., 1962).
The thyroid activity in rats exposed to 2.45 GHz microwaves at
1 mW/cm2 for 8 h per day for 8 weeks was studied by Milroy &
Michaelson (1972). No structural or functional changes were detected,
other than those that could be attributed to microwave-induced thermal
stress. In contrast, Baranski et al. (1973) reported that rabbits
exposed to 10 cm microwaves at 5 mW/cm2 showed increased thyroid
activity. Mikolajczyk (1977) suggested that these differences in
results were due to the experimental procedure and conditions rather
than the differences in species.
When rats were exposed to 2.45 GHz microwaves at 10, 15, 20, and
25 mW/cm2 for 4, 16, and 60 h (i.e., 64 h with two 2-h breaks),
Parker (1973) found that the iodine-concentrating ability of the
thyroid serum, protein-bound iodine levels, and thyroxine increased
slightly at 10 mW/cm2, but decreased at 20 and 25 mW/cm2 during
the 16-h exposure. Exposure at 15 mW/cm2 for 60 h resulted in a
decrease in protein-bound iodine and thyroxine, and a decrease in the
ability to concentrate iodine.
When male rats were exposed to 2.87 GHz radiation at 10 mW/cm2
for 6 h per day, 6 days per week for 6 weeks, there were no
significant differences between the average body and organ weights of
irradiated and control animals (Mikolajczyk, 1977). Although the
levels of growth hormone in the anterior pituitary were the same in
both groups of rats, a significantly higher level of luteinizing
hormone (LH) was found in the irradiated animals. It was suggested
that changes in the LH activity might be due to the influence of
microwave exposure on the pituitary, or on hypothalamic function or on
both.
Various animal studies in which neuroendocrine effects have been
reported following exposure to low intensity fields are summarized in
Table 14. Baranski & Czerski (1976) state in their review of endocrine
effects that it is extremely difficult to sum up the evidence. All
aspects of microwave interactions reported need further investigation
concerning both the cause and dose dependence of the effects described
and the mechanisms involved. However, it could be stated that:
(a) Microwave radiation induces endocrinological changes that may
be due to stimulation of the hypothalamic-hypophyseal system, through
thermal interaction at the hypothalamus, or immediately adjacent
levels of organization, the pituitary, the particular endocrine gland,
or the end-organ.
(b) Since the neuroendocrine system is homeostatic, transient
neuroendocrinological changes should not be equated with pathological
alterations.
(c) Sufficient data are available to indicate that the response of
the neuroendocrine system to microwaves depends on the frequency,
power density, the duration of exposure, and the part of the body
exposed.
(d) The nonuniform distribution of microwave energy within the
body seems to be an important factor affecting the response of the
neuroendocrine system.
(e) Several components of the neuroendocrine system are critically
sensitive to environmental temperature, thus low-power density,
microwave-induced effects could result from sensitivity to small
changes in temperature.
(f) From available data, it would seem that direct interaction of
microwaves with components of the neuroendocrine system cannot be
excluded.
Table 14. Neuroendocrine effects of exposure to low intensity fieldsa
Independent Dependent Experimental Results and comments Reference
variables variables subject
10 cm, cw; endocrine rats Increase in gonadotropic Mikolajczyk (1972)
0.01, 1, 3, gland hormone (in vivo) tropic hormones
10, 20 & 150 levels followed by decrease
mW/cm2; 1 h/day, 18 h after exposure at
single or repeated 10 Mw/cm2 or greater
exposures intensities; alteration
in hypothalamic function
governing the follicle-
stimulating hormone (FSH)
and luteinizing hormone
(LH) release from
pituitary; no changes
in corticosteroid content
of adrenals or
blood at 10 mW/cm2
for 15, 30, or 60 min.
10 cm, cw; adrenal rats Initial decrease in Leites &
100 mW/cm2 alterations (in vivo) Sudan III stain-positive Skuricina (1961)
10 min exposure/ lipids, birefringent
day for 14 days substances, &
ascorbic acid; Increase
in all variables
during course of exposure
return to normal 2 weeks
after exposure.
Table 14 (Cont'd)
Independent Dependent Experimental Results and comments Reference
variables variables subject
decimetre waves, adrenal cortex rats No effect on serum Nikogosjan (1962)
40 mW/cm2; 1-h alterations, (in vivo) Na+ or K+; Increase
daily exposure, serum In Ca+2 and C--1 In
prolonged electrolytes serum and urine.
duration.
15 mW/cm2; 60 h neuroendocrine rats Transient changes Michaelson et al.
up to 60 mW/cm2; responses (in vivo) in plasma corticosterone, (1977) and Lotz &
up to 2 h growth hormone, and Michaelson (1978)
thyroid hormone levels
(20-30 mW/cm3 seemed
to be the transitional
range for stimulation
of pituitary --
adrenal activation);
noted effects correlated
with temperature
increases in endocrine
gland.
2.45 GHz, cw; thyroid rats No structural or Milroy &
1 mW/cm2; function (in vivo) functional changes Michaelson (1972)
continuous exposure, other than those
8 wk; 10 mW/cm2, attributable to
8 h/day for 8 wk thermal stress.
Table 14 (Cont'd)
Independent Dependent Experimental Results and comments Reference
variables variables subject
2.45 GHz, cw; thyroid rats 23% decrease in Parker (1973)
15 mW/cm2 60 h function (in vivo) protein-bound iodine
exposure and 55% decrease
in serum thyroxine.
2.86-2.88 survival rats time, Survival time of Mikolajczyk (1974)
GHz, cw; endocrine (in vivo) hypophysectomized
10-120 mW/cm2 function rats increased at
120 mW/cm2; 2-week
habituation before
exposure alterations in
corticosterone
levels; daily exposures
at 10 mW/cm2 for 1
month did not alter
gonadotropins (LH
and FSH) but single
exposures induced
detectable alterations.
10 cm, cw; carbohydrate rabbits Changes in serum Baranski et al.
5 mW/cm2 (free metabolism, (in vivo) pyruvic and lactic (1967)
field exposure) skeletal acid; decrease in
muscle skeletal muscle
metabolism glycogen; altered
electromyography indicative
of changes in
muscle metabolism;
altered carbohydrate
metabolism.
Table 14 (Cont'd)
Independent Dependent Experimental Results and comments Reference
variables variables subject
10 cm, cw; thyroid rabbits Increased radioiodine Baranski et al.
5 mW/cm2 function (in vivo) uptake, histological (1973)
repeated exposure & electronmicroscopic
signs of thyroid
hyperfunction.
10 cm, cw; adrenal rabbits Decrease 17-hydroxy- Lenko et al. (1966)
50-60 mW/cm2, function (in vivo) corticosteroid in
4 h/day urine, first 20 exposures;
return to normal
at day 10 due
to adaptation; no
changes in 17-hydro-
xycorticosteroid in
urine.
metre & decimetre endocrine dogs, rabbits Increased adrenal Syngajevskaja
waves; 70 mW/ function (in vivo) ascorbic acid et al. (1962)
cm2; 30 min concentration following 70
mW/cm2, decrease following
5 mW/cm2, thermal
intensities suppress
pituitary & adrenal
functions; low-intensity
exposure stimulates.
Table 14 (Cont'd)
Independent Dependent Experimental Results and comments Reference
variables variables subject
1.24 GHz, pw; thyroid dogs Increased radioiodine Howland &
360 Hz pulse alterations (in vivo) uptake 4-25 days Michaelson (1959)
repetition rate; after exposure: radio-
2 ms pulse, 50 iodine uptake
mW/cm2; average increased 3-4 years
power; 6 h/day after single 100 mW/
for 6 days cm2 exposure to 1.28
GHz, pw.
2.45 GHz, cw; neuroendocrine dogs Transient increased Michaelson et al.
20-40 mW/cm2; responses (in vivo) mean plasma (1977b)
2 h corticosterone levels,
correlated with mean
colonic temperature.
a Adapted from: Cleary (1978).
7.4 Nervous System and Behavioural Effects
Microwave radiation effects on the central nervous system and
behaviour have been the subject of most controversy in the whole field
of bioeffects. Czechoslovak, Polish, and Soviet investigations on this
subject commenced in the early fifties and have been the source of
most of the reports on the effects of microwaves on man. Animal
studies and clinical and industrial surveys in Czechoslovakia, Poland,
and the USSR have been summarized by Marha et al. (1971), Baranski &
Czerski (1976), and Presman (1968), respectively. The basic assertion
is that exposure to microwaves at low power densities results in
neurasthenic disorders in man. Subjective complaints such as headache,
fatigue, weakness, dizziness, moodiness, confusion, and nocturnal
insomnia have been reported. In small experimental animals, chronic
and repeated exposures at incident power densities of 10 mW/cm2 or
less have been reported to lead to disturbances in conditioned
reflexes and to behavioural changes (Kholodov, 1966; Presman, 1968;
Petrov et al., 1970; Prey, 1971, 1977; Marha, 1971; Lobonova, 1974;
Galoway, 1975; Hunt et al., 1975; Serdjuk, 1977; Cleary, 1978).
Studies of microwave/RF exposure effects on conditioned and normal
reflexes, as well as on behaviour, were carried out on mice, rats,
guineapigs, rabbits, dogs, monkeys and in some instances on birds
(Romero-Sierra et al., 1974; Bigu-del-Blanco et al., 1975; Bliss &
Heppner, 1977).
Numerous reports of the sensitivity of the human CNS to low level
microwave exposure have stimulated interest in the subject with a
consequent increase in studies on microwave effects on the animal CNS
(Cleary, 1977). Investigations have been conducted at various levels
of CNS organization and range from studies of isolated nerves (McRee &
Wachtel, 1977) to behavioural studies in primates (De Lorge, 1976,
1979). These studies were established to determine if the effects were
thermally-induced or were the result of the direct action of
microwave-energy on the CNS. The results of many studies can be
explained by the nonuniform distribution of thermal energy and/or
thermal gradients, but the results of others such as the increase in
calcium efflux from cerebral tissue, due to specific amplitude
modulation are difficult to explain on the basis of heating.
Disturbances in the bioelectric function of the chick forebrain
with calcium efflux were observed following in vivo exposure to
147 MHz radiation, amplitude modulated at 9-20 Hz (Bawin et al.,
1975). These effects could not be obtained when the frequency of
amplitude modulation was between 6 and 9 Hz or between 20 and 35 Hz. A
20% increase in calcium was also observed by Kaczmarek & Adey (1974)
in the cat brain after in vivo exposure to 10 ms pulsed radiation at
200 Hz, 20-50 mV/cm2. Further research is needed since these effects
may depend on a direct interaction of electromagnetic fields with the
cellular membrane (Grodsky, 1975; Straub, 1978; Kolmitkin et al.,
1979).
Blackman et al. (1979) recently confirmed the work of Bawin and
Adey and their coworkers, in finding that calcium efflux from brain
tissue depended on amplitude modulation frequency and power levels.
Increased calcium efflux appeared at amplitude modulation frequencies
around 9 Hz, peaked from 11-16 Hz, and disappeared above 20 Hz as
shown in Fig. 14. It can be said that a "frequency window" exists for
this phenomenon. Calcium efflux appears at 0.5 mW/g, reaches higher
values at 0.75 mW/g and decreases at 1.0 mW/g. Thus, it can be said
that "power windows" also exist. These may shift with frequency
(Blackman et al., 1979).
The electrical activity of the brain, measured by means of an EEG,
may be influenced by a wide variety of exposure regimes. Acute single
exposures to 40 mW/cm2 or more, induce transient changes in EEG
patterns. Early experimentation in this area has been summed up by
Kholodov (1966). Long-term, repeated exposures of dogs, cats, rabbits,
rats, frogs, and mice at power densities between 2 and 5 mW/cm2 were
reported to lead to alterations, such as the desynchronization of
basal rhythms and later a flattening in EEG tracings (Baranski &
Edelwejn, 1968; Bychkov & Dronov, 1974; Bychkov et al., 1974; Gillard
et al., 1976). However, these earlier reported effects are
questionable since experiments were carried out using EEG electrodes
or wires that significantly perturbed the field.
Mice, rats, and rabbits subjected to long-term, low or
medium-level (about 1-5 mW/cm2) exposure were reported to show an
increased susceptibility to convulsant drugs (Baranski & Edelwejn,
1968; Servantie et al., 1974, 1975; Krupp, 1977). Detailed analyses of
EEG data and results of pharmacological studies indicate that the
reticular formation of the midbrain is the structure in which exposure
to microwaves and RF may induce effects at low incident power density
levels.
The mechanism of changed susceptibility to drugs acting on the
nervous system, particularly convulsant drugs, after repeated
microwave exposures is unclear. On the other hand, as the action of
many drugs is well understood, the phenomenon may serve to clarify
mechanisms of action of microwave and RF radiation on the nervous
system (Czerski, 1975). The phenomenon has practical implications in
the case of the medication of microwave workers.
Structural changes in the nervous tissue of rabbits and hamsters
which were demonstrable by electron and light microscopy, were
reported following single exposures to 2450 MHz microwaves at power
densities of 25-50 mW/cm2 (Baranski, 1967; Baranski & Edelwejn,
1979; Albert & De Santis, 1975; Albert, 1979). In their study on
rabbits subjected to single or repeated exposures to continuous or
pulsed microwaves (2950 MHz), Baranski & Edelwejn (1974) did not find
any effects on acetylcholinesterase activity after long-term exposure
(2 h/day for 3-4 months to 3.5-5 mW/cm2).
Brain hyperaemia, pyknosis, and vacuolization of nerve cells were
observed in rats repeatedly exposed for 75 days to 3- and 10-cm
microwaves at high power densities (40-100 mW/cm2) (Tolgaskaya et
al., 1962; Tolgaskaya & Gordon, 1973). These effects were less
pronounced following exposures at 10-20 mW/cm2 and with exposure to
3-cm microwaves compared with 10-cm microwaves at the same power
density. The effects were reversible, several days after termination
of the experiment.
The blood-brain barrier of rats may be affected by pulsed and
continuous wave microwave radiation at 1.2 GHz (Frey et al., 1975). A
single exposure of 30 min at an average power density of 0.2 mW/cm2
pulsed and 2.4 mW/cm2 continuous wave radiation led to an increase
in permeability. In another study on rats, Oscar & Hawkins (1977)
found temporary alterations in permeability following single 20-min
exposures to 1.3 GHz radiation at power densities of about 1 mW/cm2
pulsed and 3 mW/cm2 cw. Many other investigators including Merrit
(1977) and Sutton & Carrell (1979) were unable to reproduce these
experimental results.
In studies by Wachtel et al. (1975), exposure of individual
neurons to 1.5 GHz and 2.45 GHz microwave radiation at a dose rate of
approximately 10 mW/g had a marked effect on the firing pattern of
Aplysia neurons. Although heating may have been partially responsible,
the authors suggest that other factors are needed to explain the
effect. Rectification of the applied field in nerve tissue could
explain the observed effects.
The threshold power density required to evoke potentials in the
brain stem of cats using nonperturbing electrodes was found to be
approximately 0.03 mW/cm2 with a peak of 60 mW/cm2 for frequencies
between 1.2-1.5 GHz (Frey, 1967).
Stverak et al. (1974) found that rats having an inherent
predisposition to epileptic seizure after sound stimulation showed
reduced sensitivity of this phenomenon following long-term (4 h/day
for 10 weeks) exposure to 2850 MHz radiation, pulsed for 10 µs,
repetition frequency 769.2 Hz, at an average power density of
30 mW/cm2.
Behavioural perturbations in rats in the form of work stoppage
have been reported by Justesen & King (1970) and Lin et al. (1979).
Exposure of hungry unrestrained rats to 2.45 GHz microwaves at a dose
rate of approximately 9 mW/g caused stoppage of work for food after
20 min of exposure in a multimode cavity (Justesen & King, 1970). With
restrained rats irradiated with near-field radiation at 918 MHz, the
threshold dose rate for the effect was 8 mW/g (Lin et al., 1979). It
was calculated by Justesen (1978) that an integral dose between 8 and
10 J/g was required for work stoppage in hungry rats, e.g., 23 min
exposure to an average power density of 20 mW/cm2 at 600 MHz
(resonant frequency for the rat) or 46 min exposure to the same power
density at 400 MHz. The work stoppage was found to be related to the
specific absorption rate, suggesting a thermal basis for the effect.
In studies by Moe et al. (1977), rats exposed for 210 h to 918-MHz
radiation at 10 mW/cm2 showed decreased locomotor activity and food
intake. This behavioural change could be attributed to thermal
loading, even though the animals were not under hyperthermic stress.
The effects on exploratory activity, swimming, and discrimination
involving a vigilance task were studied in rats exposed to 2.45 GHz
pulsed radiation (Hunt et al., 1975). A dose rate of 6 mW/g caused a
moderate decrease in the level of exploratory activity and swimming
speed. The results were attributed to fatigue from thermal
overexposure, since the effect on vigilance discrimination was
observed to be directly related to induction of and recovery from
hyperthermia. Nearly lethal radiation (11 mW/g) initially produced a
marked degradation in performance, but the rats returned to the
trained level of proficiency after 1 h.
Microwave radiation was found to affect the behaviour of rats
conditioned to respond to multiple schedules of reinforcement (Thomas
et al., 1975). Exposure for 30 min to 2.86- and 9.6-GHz pulsed
radiation, and to 2.45-GHz cw radiation just before experimental
sessions at power densities exceeding 5 mW/cm2 caused significant
alterations in behaviour.
Roberti et al. (1975) did not find any difference in the
spontaneous motor activity of rats after exposure for periods
totalling 408 h to 10.7- and 3-GHz microwaves at power densities
ranging from 0.5 to 26 mW/cm2. Classical Pavlovian methods were used
by Svetlova (1962) and Subbota (1972) to investigate reflex and
conditioned reflex actions in microwave-irradiated dogs, by
determining the time of initiation of saliva secretion following the
conditioning stimulus, the latency time, and the number of drops
secreted. After lateral exposure to 10-cm microwaves for 2 h at power
densities ranging from 1-5 mW/cm2, the intensity of the response
increased on the opposite side, and the latency time was shortened.
However, following 70 h of exposure in 35 days (2 h/day), the
conditioned responses became identical to those before irradiation
showing that a gradual adaptation of the dogs' responses to successive
microwave exposures occurred.
Galloway (1975) investigated the effects of 2.45 GHz cw microwave
exposure on discrimination and acquisition tasks in trained rhesus
monkeys. The heads of the animals were exposed directly with energy
deposited at rates ranging from 5 to 25 W (for a 1.2 kg head the
resulting average dose rate was between 4 mW/g and 21 mW/g). Before
testing, the monkeys were given a dose of 2.5 J/g over 2 min.
Convulsions occurred in all animals irradiated at 25 W and in some at
15 W, an integral dose approaching 25 J/g (the dose required to
produce convulsions (Justesen, 1978)) was given. It is apparent that
hot spots were produced in the monkey's brains to induce this effect.
Exposure to 10 W for 5 days, for 40 min per day did not produce any
performance deficit, even in animals suffering from skin burns and
severe convulsions caused by exposure to high power radiation.
The performance of a vigilance task was investigated in rhesus
monkeys after whole body exposure to 2.45 GHz far-field radiation.
Behaviour was not disrupted provided that increases in colonic
temperature did not exceed 1°C. With a 1-h exposure, the threshold of
behavioural disruption was 70 mW/cm2 (De Lorge, 1976).
Exposure to continuous wave microwave radiation of 1.2 GHz at
average power densities of 10-20 mW/cm2 did not affect skilled motor
performance in monkeys even when the animals were positioned for
maximum energy deposition in the brains and subjected to three 2-h
periods of exposure (Scholl & Allen, 1979).
A number of studies including some of those already discussed and
others for comparison are summarized in Table 15. The results obtained
by different investigators vary according to exposure conditions and
the end-point investigated. Interpretation of these observations is
difficult since many observations are either controversial or
contradictory. Data tend to be better substantiated at power densities
above 5-10 mW/cm2.
In 1961, Frey reported the sensory effect of "microwave hearing".
Man perceives an audible clicking or buzzing sensation on exposure to
pulsed radiation at low power densities. He (Frey, 1971) considered
that the effect was caused by direct neural stimulation but later
studies by Foster & Finch (1974) and Chou et. al., (1977) have
strongly indicated that an electromechanical interaction occurs due to
thermal expansion. The threshold of microwave hearing is approximately
10 mJ/g per pulse and is independent of the pulse width for pulses of
less than 30 microseconds (Guy et al., 1975a). Microwave hearing is
now thought to be caused by a small but fast rise in temperature
which, by thermal expansion, generates a wave of pressure exciting the
cochlea.
To summarize, it can be stated that studies on the effects of
microwaves/RF radiation on the nervous system indicate that exposure
at low-power densities appears to induce detectable changes in some
cases (Cleary, 1977). While there seems to be evidence that, at
sufficiently high intensities (above 1-5 mW/cm2), nonuniform heating
of various critical organs takes place in experimental animals, it is
not possible at present to exclude other mechanisms. Furthermore, it
is difficult to evaluate the significance of microwave-induced
behavioural effects because of the general lack of quantitative
correlations between thermal effects at low power densities and
responses at the physiological or psychological levels of analysis
(Cleary, 1977).
Table 15. Neural effects of exposure to low-intensity fields
Independent Dependent Experimental
variables variables subject Results and comments Reference
200 Hz; 10 ms cerebral cat-direct 20 % increase in Kaczmarek &
pulsed field; Ca+2 cortical Ca+2 efflux Adey, (1974)
20-50 mV/cm efflux stimulation from neurons.
(in vivo)
147 MHz. AM cerebral chick fore- Increase in Ca +2 Bawin et al.
modulated at Ca+2 brain from neurons; no (1975)
6, 9, 11, 16 Hz; efflux (in vivo) change from unmodulated
1-2 mW/cm2 fields; maximum
(closed irradiation rate of efflux
system) at 11 Hz and alterations
in neuron firing
patterns at intensity
equivalent to 10 mW/cm2
free field exposure.
ELF fields, cerebral isolated chick Suppression in Bawin &
1-75 Hz; 0.5 Ca+2 and cat Ca+2 release Adey (1976)
to 1 V/cm efflux cerebral from neurons;
(closed system tissue biphasic intensity &
irradiation) (in vivo) frequency dependence;
maximum effect at
6 and 16 Hz; 0.1 and
0.56 V/cm.
1.5 and 2.45 GHz, electrical aplysia ganglia Effects attributed to Wachtel et al.
cw and pw (closed activity of (in vivo) ganglionic warming, (1975)
irradiation system) individual but effects not produced
neurons by non-radiation heating.
Table 15 (Cont'd)
Independent Dependent Experimental
variables variables subject Results and comments Reference
2.45 GHz, cw functional spinal cord of Alteration in evoked Taylor &
alterations cat potentials also produced Ashlemen, (1975)
in neuronal (in vitro) by non-radiation
elements heating but
with change in timing.
3 GHz, pw; electrical rat 10-day exposure resulted Servantie et al.
5 mW/cm2; pulse activity of (in vivo) in synchronization (1975)
repetition rate cortical of electronic
500-600 Hz neurons frequency; synchronization
(free field persisted for
exposure) hours after exposure.
2.45 GHz, cw, synaptic rabbit vagus No changes other Chou & Guy
0.3-1500 mW/g, transmission; nerves, superior than those thermally- (1975)
pw; 0.3-2.2 × neural function cervical induced.
1055 mW/g ganglia; rat
temperature diaphragm
controlled muscle
exposure (closed (in vitro)
system irradiation)
3.1 GHz; pw axonal transport rabbit vagus No effects. Paulsson et al.
10-400 W/kg & microtubules nerve & brain (1977)
Mean 5 × 104 to extracts
2 × 106 W/kg peak (in vivo)
temperature controlled
exposure
(free space irradiation)
Table 15 (Cont'd)
Independent Dependent Experimental
variables variables subject Results and comments Reference
decimeter waves; neurotransmitter rabbit Decreased acetyl- Syngajevskaja,
0.5 mW/cm2 (free release (in vivo) cholinesterase (ACHE) et al. (1962)
space irradiation) activity.
10 cm. 0.5 mW/cm2 neurotransmitter rabbits & No alteration caused Baranski (1967)
(free space release guineapigs by 8-month exposure
irradiation) in brain (in vivo) to 1 mW/cm2; with
3-h exposure to 3.5
mW/cm2, no cw
effect but pw decreased
AChE activity
in guineapigs; after
4 months exposure,
there was a decrease
with cw and an
increase with pw;
midbrain most affected;
lipid & nucleoprotein
metabolism altered in
rabbit.
1.6 GHz; 80 neurotransmitter rats 10-min exposure led Merrit et al. (1976)
mW/cm2 environmental release (in vitro) to 4 °C rectal temperature
temperature in brain rise in irradiated
(free space and heated controls;
irradiation) hypothalamic evarternol
(nore-pinephrine)
decreased in both groups;
serotonin decreased in
hippocampus of irradiated
animals only.
Table 15 (Cont'd)
Independent Dependent Experimental
variables variables subject Results and comments Reference
1.7 GHz, cw; histological Chinese 30-120 min exposure Albert &
10 and 25 mW/ alterations hamster led to cytopethological DeSantis (1975)
cm2 (free space in brain (in vitro) effects In hypothalamic
irradiation) and subthalamic neurons; no
effect on other brain
regions or on glial
cells; no repair evident
dent 14 days after
exposure.
960 MHz, cw; heart rate isolated Bradycardia due to Tinney et al.
2-10 mW/g (closed turtle heart alteration in neuro- (1976)
system irradiation) (in vitro) transmitter release;
biphasic intensity
response.
10.5 cm, cw; passive and skeletal Differential effect Portela et al.
0.5-10 mW/cm2; dynamic muscle, South of microwave exposure (1975)
temperature-controlled electric American frog on dependent
exposure (free parameters (in vivo) variable time constants;
space irradiation) muscle cells of summer
frogs more sensitive than
those of winter frogs.
3 & 10.7 GHz, behavioural rat No effects on spontaneous Roberti et al.
cw; 0.-526 mW/ modification (in vivo) motor activity. (1975)
cm2 408-h exposure (spontaneous
(free field motor activity)
exposure)
Table 15 (Cont'd)
Independent Dependent Experimental
variables variables subject Results and comments Reference
9.4 GHz, pw; behavioural rat Control results: Gillard et al.
2.3 mW/cm2 & modification (in vivo) decrease in locomotor (1976)
0.7 mW/cm2 average; (free field activity, & vigilance:
2 week exposure spontaneous increase in exploratory
(free field behaviour) activity; exposed
exposure) results: increased
exploratory activity
(slower than controls);
increase then decrease
in vigilance, uniform
locomotor.
2.45 GHz, pw; behavioural rat Dose-dependent Thomas et al.
5, 10, 15 mW/cm2, modification (in vivo) increase in the (1975)
30-min exposures (fixed frequency of premature
(free field exposure) consecutive switching alteration
number switching (in the perception).
frequency)
2.45 GHz, cw; behavioural rhesus monkey Vigilance performance De Lorge (1976)
4-72 mW/cm2; modification (in vivo) not affected by
30, 60, 120-min (auditory exposure.
exposures (free vigilance
field exposures) task)
Table 15 (Cont'd)
Independent Dependent Experimental
variables variables subject Results and comments Reference
2.45 GHz, cw; behavioural rhesus Convulsions induced Galloway (1975)
2-min exposure, modification monkey at 15 and 25 W;
5-25 W output (discrimination (in vivo) irradiation 40 min/
(applicator and repeated day for 5 days did
exposure of acquisition) not produce any
head) behavioural effects
at less than 15 W;
no low-intensity
effects.
9.3 GHz, cw; amplitude of rabbit Atypical arousal phenomena, Goldstein &
0.7-2.8 mW/cm2, cortical brain (in vivo) 3-12 min after exposure, Sisko (1974)
5-min exposure waves in followed in 3-5 min
(free field exposure) anaesthseized by longer period of
animals arousal; atypical
(pentobarbital) behaviour.
2.45 and 1.7 duration of rabbit Dose-dependent analeptic Cleary &
GHz, cw & pw; pentobarbital- (in vivo) effect. Wangemann (1976)
5-50 mW/cm2 induced sleeping
(free field exposure) time
Table 15 (Cont'd)
Independent Dependent Experimental
variables variables subject Results and comments Reference
3 GHz, pw; effects of mice (in vivo) Exposure delayed Servantie et al.
5 mW/cm2 (free drugs on CNS rats (in vivo) onset of pentetrazol- (1974)
field exposure) and (in vitro) induced convulsion
during first 15 days
of exposure, reduced
latency after 15 days;
decreased susceptibility
to curare-like
drugs in in vivo and
in vitro systems.
Occupational CNS drug human Altered EEG patterns Edelwejn &
microwave & tolerance subjects and convulsions in Baranski (1966)
RF exposure (cardiazole) (in vivo) workers with over 3 Baranski &
years microwave Edelwejn (1968)
exposure (similar results
reported in rabbits).
occupational CNS functional human Transient subjective Petrov (1970)
microwave & disorders subjects complaints during
RF exposure (in vivo) first year of exposure;
phasic adaptation
after 1 year; objective
symptoms of neurovegetative
disturbances after 5
years of exposure
(acrocyanosis, hyper-
hydrosis, dermographism,
hypotonia tremors.
Table 15 (Cont'd)
Independent Dependent Experimental
variables variables subject Results and comments Reference
3.1 GHz; pw, histological rabbit Cytopathological effects Paulsson et al.
55 mW/cm2, repeated alterations in (in vivo) in the plexiform (1979)
or single retina layers of retina,
1-h exposure no effects on photo-
(free space receptors; alterations
irradiation) persisted for 3 months
after radiation.
From: Cleary (1978).
7.5 Effects on the Blood Forming and Immunocompetent Cell Systems
Studies have been conducted on the effects of microwave radiation
on blood and the immunocompetent system, but the results are
frequently contradictory and the reasons for the discrepancies are not
always easily identified. For example, in 1962, Prausnitz & Susskind
irradiated 100 mice with 9270 MHz microwaves at 100 mW/cm2 for
9.5 min daily over a period of 59 weeks and reported an increase in
white blood cells accompanied by lymphocytosis. It was reported that
leukaemia occurred in 35% of exposed mice, compared with 10% of the
controls. However, it appears that no attempt has been made to
replicate these studies.
A decrease in erythrocytes, leukocytes, and haemoglobin in mice
was observed by Gorodeckij, ed. (1964) immediately after exposure to
10 GHz at 450 mW/cm2 for 5 min, and 1 and 5 days later, while
recovery was evident after 10 days. The influence of microwaves on the
response of immunocompetent lymphocytes was investigated in mice by
Czerski (1975). The animals were exposed to 2.95 GHz microwaves at
0.5 ± 0.2 mW/cm2 for 2 h per day, 6 days per week for 6 and 12
weeks. During the 2-h exposure, the animals were deprived of food and
water and were located in separate cages. After exposure, the animals
were immunized with antigen and the immune response determined by the
number of antibody-forming cells in the lymph nodes. Significant
differences were found between the control group and the group exposed
for 6 weeks, but not the group exposed for 12 weeks. The author
attributed this result to adaptation. In nonimmunized irradiated mice
there was an increased number of lymphoblasts in lymph-node cells, but
no differences in the number of plasmocytes.
Blast transformation of human lymphocytes in vitro was observed
by Stodolnik-Baranska (1967, 1974) after exposure to 2950 MHz
microwaves at power densities of 7 and 20 mW/cm2. However,
Smialowicz (1977) was unable to detect any differences between the
blastogenic responses of microwave-exposed (2450 MHz, 19 W/kg for
1-4 h) and control mouse splenic lymphocytes activated with various
mitogens in vitro.
The effects on haemopoietic-stem cells in mice of exposure to
2.45 GHz microwaves at 100 mW/cm2 for 5 min were investigated by
Kotkovská & Vacek (1975). The response appeared to occur in 2 stages.
In the first, the number of leukocytes in the blood increased and both
bone marrow and spleen cell numbers decreased for 3-4 days following
exposure. In the second stage, the number of nucleated cells in the
spleen and the total number of cells in the femur, as detected by
incorporation of 59Fe, increased until the twentieth day after
exposure. The incorporation of 59Fe in the spleen decreased to 78%
of the control value 24 h after exposure and increased to 50% after 14
days.
When Lin et al. (1979) studied the effects on mice of single and
repeated exposures to 148 MHz radiation at 1 mW/cm2 for 1 h per day,
5 days per week for ten weeks, they did not find any significant
changes in the blood.
In studies on 3 strains of rats, a 7-h exposure to 24 GHz
microwave radiation at 20 mW/cm2 induced significant leukocytosis,
lymphocytosis, and neutrophilia with recovery in 1 week; after a
10-min exposure at 20 mW/cm2 or a 3-h exposure at 10 mW/cm2,
recovery occurred in 2 days (Deichman et al., 1964). The changes
observed were strain-dependent because in 2 strains the number of
leukocytes, erythrocytes, and neurophiles increased, while in one
strain it decreased.
Decreases in lymphocytes, erythrocytes, and leukocytes, and
increases in granulocytes and reticulocytes were observed in rats by
Kitsovskaja (1964) after 3 GHz exposure at 40 mW/cm2 (15 min per day
for 20 days) and 100 mW/cm2 (5 min per day, for 6 days). Exposure at
10 mW/cm2 (1 h per day for 216 days) resulted in decreases in total
WBC and lymphocytes and an increase in granulocytes with no changes in
other blood components. However, in a study on rats exposed to 2.4 GHz
microwaves at 5 mW/cm2 (1 h per day for 90 days), Djordjevic et al.
(1977) did not observe any significant differences in the haematocrit,
mean cell volume, and haemoglobin between the exposed and control
groups during 90 days of exposure and for 30 days afterwards.
Furthermore, there were no significant differences in the number of
leukocytes, erythrocytes, lymphocytes, and neutrophiles.
Smialowicz et al. (1977) completed a comprehensive study on rats
chronically exposed to 425 MHz radiation at 10 mW/cm2 (SAR,
3-7 mW/g) and to 2.45 GHz radiation at 5 mW/cm2 (SAR, 1-5 mW/g). The
rats were exposed in utero and for the first 40 days of life for 4 h
per day. The only change in the haemopoietic or immunocompetent
systems was observed in the response of lymphocytes to mitogen.
The effects on guineapigs and rabbits of prolonged intermittent
exposures to 3 GHz radiation at 3.5 mW/cm2 for 3 h per day over
3 months were investigated by Baranski (1971). Increases in absolute
lymphocyte counts in peripheral blood, and abnormalities in nuclear
structure and mitosis in erythroblast cells in the bone marrow, and in
lymphoid cells in lymph nodes and spleen, were found. Rabbits exposed
at 3 mW/cm2 (2950 MHz continuous and pulsed) for 2 h per day, for 27
and 79 days showed a decrease in erythropoiesis, as determined by
59Fe uptake. Pulsed radiation was found to be more effective than
continuous radiation at the same power level (Czerski et al., 1974a).
The effects on blood serum in rabbits exposed to 2.45 GHz
continuous and pulsed radiation at 5, 10, and 25 mW/cm2 for 2 h were
investigated by Wangemann & Cleary (1976). Changes in the blood
chemistry of animals irradiated at the three power densities were
found to be consistent with a dose-dependent response to thermal
stress. Out of the ten serum components that were analysed,
statistically significant increases were observed in serum glucose,
blood urea nitrogen, and uric acid. Dose-dependent transient increases
returned to normal levels during the week following exposure. No
differences in the animal's responses to cw and pulsed radiations
(10-micro-second duration, peak power 485 mW/cm2) were found at the
same average power density.
Dogs were exposed to 1285 MHz, 2.8 GHz and 24 GHz at power
densities between 20 and 165 mW/cm2 (Michaelson et al., 1964, 1971).
Following exposure to 1285 MHz radiation at 100 mW/cm2 for 6 h, a
marked increase in leukocytes and neutrophils was found. After 24 h,
the neutrophil count continued to increase but the lymphocyte and
eosinophil counts decreased. Neutrophil counts after exposures at 50
and 20 mW/cm2 (1285 MHz) did not differ significantly from those of
control animals. A decrease in lymphocytes was noted after the
exposures at 100 mW/cm2 and 50 mW/cm2, but not after that at
20 mW/cm2 (Michaelson et al., 1971). Haematological examination of
the dogs for 12 months after exposure at 20 mW/cm2 did not reveal
any end points that differed from the control groups.
A number of studies are listed in Table 16 with details of
exposure conditions and results of microwave-induced changes in the
haemopoietic and immunocompetent cell systems.
This section on the effects of microwaves on the blood forming and
immunocompetent cells can be summarized as follows:
(a) Changes in the red and white blood cell counts seem to depend
on the dose of microwave energy applied. In most of the studies
reporting positive findings, the effects seem to result from thermal
stress.
(b) Repeated exposures to 5 mW/cm2 or below do not appear to
affect the peripheral blood picture. Effects reported from exposures
to 15 mW/cm2 or more, depending on the biological system exposed,
tend to be reversible following termination of exposure.
(c) The response of the haemopoietic system to microwave radiation
is significantly different from that to exposure to elevated ambient
temperatures, even when both result in the same increase in rectal
temperature. This can be attributed to the nonuniform deposition of
microwave energy in the body, and the greater depth and rate of
heating.
(d) There is evidence that lymphocyte stimulation and effects on
response may occur under certain experimental conditions, especially
after exposure to pulsed radiation for repeated or prolonged periods
at sufficiently high power densities.
Table 16. Microwave-induced effects induced in the blood-forming and immunocompetent cell systemsa
Radiation Intensity Exposure
Frequency (mW/cm2) duration Species Results Reference
(GHz)
3 1,5 15,30 granulocyte Liberation of hydrolases Szmigielski
cells (1 mW/cm2) cell death (according to
in culture (5 mW/cm2; 60 min) Cleary (1978))
lysosomal enzyme release
(5 mW/cm2; 60 min).
2.95 0.5 2 h/day for mouse Lymphoblasts in lymph Czerski
8 days/week nodes, lymphoblastoid (1975 b)
for 6 and/ transformation during first
2 weeks 2 months and 1 month
after exposure.
2.45 100 5 min mouse Leukocyte (2 maxima) total Rotkowska &
cell volume in the bone Vacek (1975)
marrow end spleen, 59Fe
Incorporation into spleen,
nucleated cells in spleen
immediately after exposure
total cell number in femur
5-7 days after exposure.
Colony forming unit numbers
of stem cells, return
to normal 12 h after
exposure.
Table 16 (Cont'd)
Radiation Intensity Exposure
Frequency (mW/cm2) duration Species Results Reference
(GHz)
3.0 3.5 4 h/day rat Leukocyte, altered nuclear Baranski
structure, altered mitotic (1971)
activity in erythroblasts,
bone marrow cells &
lymphatic cells in lymph
nodes & spleen.
24 220,10 varied rat Leukocyte lymphocyte Deichman
neutrophil, all cell counts et al. (1959)
returned to normal in
7 days.
2.95 3 2 h/day for rabbit Erythrocyte production Czerski et al.
37 days alterations in circadian (1974 a)
pw & cw, rhythms in haemapoietic
2 h/day for cell mitosis.a
79 days cw
2.45 3,10,25 2 h rabbit Serum glucose blood urea Cleary &
nitrogen, uric acid, all Wangemann
values return to normal (1978)
7 days after exposure.
1.28, 100-165 7 h dog Maximum increase in 59Fe Michaelson
2.8 incorporation 45 days et al. (1951)
after exposure.
a From: Bramall (1971).
7.6 Genetic and Other Effects in Cell Systems
Investigations of biological systems such as cells in culture are
conducted to gain an understanding of basic mechanisms of interaction.
Although, these systems are less complex and the dosimetry can be
better quantitated than in animal studies, the results have to be
interpreted carefully in assessing potential health hazards to man.
Microwave exposure has been reported to produce chromosomal
aberrations (Janes et al., 1969; Mykolajkzyk, 1970; Yao & Jiles, 1970;
Baranski et al., 1971; Yao, 1971; Czerski et al., 1974b) and mitotic
alterations (Baranski et al., 1969; Mykolajkzyk, 1970; Baranski et
al., 1971; Baranski, 1972; Czerski et al., 1974) in cells.
Yao & Jiles (1970) studied the effects of microwave radiation on
cell proliferation and on the induction of chromosomal aberrations in
cultured rat kangaroo cells. Cells were exposed to 2.45 GHz radiation
in the near field at 1 W/cm2 and 5 W/cm2 and in the far field at
0.2 W/cm2. Exposure to 0.2 W/cm2 for 1 min caused increased cell
proliferation, but after 30 min, it decreased. Exposure at higher
power densities significantly reduced the rate of proliferation,
Exposure at 5 W/cm2 induced chromosomal aberrations, but it is
evident that high temperatures were involved in this result, since the
energy absorption rate measured was 15.2 mW/g.
Chromosomal aberrations and changes in the duration of particular
phases of mitosis (mitotic abnormalities) were reported by Baranski et
al. (1969, 1971) in human lymphocyte cultures and cultures of monkey
kidney cells following exposures at 3 and 7 mW/cm2 to 10 cm pulsed
and cw microwaves. Mitotic disorders in the lymphocytes of guineapigs
and rabbits were also found following exposure to 3 GHz at
3.5 mW/cm2 for 3 h per day over 3 months (Baranski, 1972).
Manikowska et al. (1979) studied 16 mice subjected to 9.4 GHz
pulsed (width 0.5 µs, repetition rate 1000 Hz) microwaves at power
densities of 0.1, 0.5, 1.0 and 10 mW/cm2 for 1 h/day for 2
consecutive weeks (5 days/week). Disturbances in meiosis were detected
at power density levels as low as 0.1 mW/cm2. This study needs
confirmation since no other studies on effects of microwaves on
meiosis could be found.
Exposure of murine splenic lymphocytes in vitro to 2450 MHz
radiation at 10 mW/cm2 (dose rate of 19 mW/g) did not result in any
changes in capacity to synthesize DNA (Smialowicz, 1977). This
technique used to assess blastic transformations of lymphocytes, was
not the same as that used by Baranski (1972), which may explain the
discrepancy in results. Elder & Ali (1975) found similarly negative
results when they exposed mitochondria of isolated rat liver to
2.45 GHz radiation at 10 and 50 mW/cm2 for 3.5 h. Furthermore, no
effects were found in oxidation of substrate, electron transport,
oxidative phosphorylation, or calcium transport.
The effects of 2450 MHz microwaves and a 43°C water bath on normal
and virus-transformed fibroblasts of mice were compared by Janiak &
Szmigielski (1977). Short-term heating by both methods resulted in
reversible changes in the active transport of potassium through the
cell membrane. Prolonged heating (over 20 min at 32°C) caused
irreversible damage and the membrane became permeable to large
molecules.
In another comparative study, Lin & Cleary (1977) did not find any
differences in the release of potassium ions, haemoglobin levels, and
the osmotic fragility of the red-cell membrane between samples exposed
to microwaves at 2.45, 3.0, and 3.95 GHz and conventionally heated
samples.
Chinese hamster ovarian cells exposed to 2.45 GHz microwaves or
treated in a water bath at the same temperature did not show any
differences in response when cell survival and sister chromatid
exchanges were the end points (Livingston et al., 1979).
In studies by Blackman et at. (1975), the colony forming ability
of Escherichia coli B was not inhibited by exposure to 1.7 GHz,
2.45 GHz, 68-74 GHz, and 136 GHz at power densities ranging from 0.3
to 20 mW/cm2. This was in contrast to an inhibitory effect of
microwave radiation at 136 GHz previously reported (Webb & Dodds,
1968). However, in a more recent study of colony-forming ability and
of alterations in the molecular structure of living E. coli B, there
were no changes in colony growth, or in molecular structure or
conformation after irradiation at frequencies between 2.6 and 4 GHz
with a specific absorption rate of 20 mW/g (Corelli et al., 1977).
Thus, it can be concluded that:
(a) Chromosomal aberrations and mitotic alterations can be
produced by microwaves at high power densities where thermal
mechanisms play a definite role. However, as there are many
conflicting reports, some doubts remain as to whether these effects
can occur at lower power densities.
(b) Studies at the cellular and subcellular level are important
for understanding basic interaction mechanisms. Chromosomal
aberrations and mitotic alterations are potential early indications of
biological changes and may reflect a response of specific tissue, but
not genetic injury in the organism.
(c) Recent studies on cell proliferation and capacity to
synthesize DNA indicate that power densities sufficient to produce
thermal damage are necessary for effects to appear. This is shown by
experiments comparing the effects of both water baths and microwave
exposure. Exposure of animals to resonant frequencies (e.g., 2450 MHz
for mice), could be expected to induce effects at low power densities
because a larger proportion of the incident radiation is absorbed and
converted into heat.
7.7 Effects on Reproduction and Development
Detrimental effects of microwaves on testicular function,
impregnation, developing embryos, and on offspring have been reported
in the literature.
Van Ummersen (1961) exposed 48-h chick embryos to 2450 MHz cw
microwave radiation through the intact shell. The power density was
20-40 mW/cm2 and exposures were given for 280-300 min, causing the
yolk temperature to rise from 37°C to 42.5°C. Abnormalities which were
observed appeared to be caused by the inhibition of cell
differentiation and growth. Development of hind limbs, tail, and
allantois was suppressed. When control eggs were incubated at 42.5°C
for the same length of time and the temperature of the eggs was the
same as that of microwave-treated eggs, no abnormalities were found.
It was concluded that the abnormalities from microwave exposure were
caused by other than thermal factors.
Mice were subjected to 2.45 GHz microwaves at the near lethal dose
rate of 38 mW/g for 10 min per day during the 11th-14th days of
gestation. There were no increases in fetal mortality or deformations
in treated animals compared with untreated controls and maze
performance was the same in both groups (Chernovetz et al, 1975).
When rats were irradiated between days 1 and 16 of pregnancy with
27 MHz radiation at power densities that caused the rectal temperature
to rise to 42°C, a variety of teratological effects related to the
developmental stage of the fetus was found. The effect on the
development of the rat of repeated exposures in utero to 2.45 GHz
radiation at 10 mW/cm2 for 5 h per day from day 3 to day 19 of
gestation was investigated by Shore et al. (1977). Two groups of
animals were exposed under different conditions of configuration in
the field. One group was placed in the exposure field in such a way
that the long axis of each animal was parallel to the electric field;
animals of the second group were placed with the long axis parallel to
the magnetic field (orientation parallel to the electric field results
in substantially greater absorption of microwave energy). No
significant differences in litter size were observed between the
control and irradiated animals. Decreases in body and brain mass were
observed in animals irradiated with the long axis of the body parallel
to the electric field.
Rats were exposed to 2.45 GHz radiation at 10 and 40 mW/cm2 for
1 h per day during critical periods of gestation, and their functional
development was studied during the 21-day period to weaning
(Michaelson et al., 1977a). Offspring of rats exposed at 40 mW/cm2
showed a significantly higher level of corticosterone during the first
24 h of life and an increase in levels of thyroxin at 14 and 16 days
of age. Thyroxin levels tended to be lower in one-week-old rats from
dams that had been exposed at 10 mW/cm2, but increased during the
second week of life. Adrenal wet mass and ratios of adrenal-to-body
mass in 7-day-old rats were significantly higher in irradiated
animals. The authors suggested that while microwave radiation might
change the developmental process and accelerate the rate of
maturation, it might also result in some deficiencies. A similar
result was found by Johnson et al. (1977) who exposed rats in utero
to 918 MHz at 5 mW/cm2 for a total of 380 h and found an increase in
body mass at birth and an acceleration in the time of eye opening.
Later a deficiency in avoidance response was observed.
Repeated exposures to 9.4 GHz at power density levels below
10.0 mW/cm2 may induce disturbances in spermatogenesis and meiosis
in mice (Manikowska et al., 1979). However, Cairnie & Harding (1979)
were unable to find any differences in the sperm counts of mice, after
in vivo exposure to 2450 MHz radiation at 20-32 mW/cm2 for 4 days
(16 h/day). Testicular damage was observed in mice exposed to 2450 MHz
at a power density of 6.5 mW/cm2 for 230 h over a 2-month period
(Haidt & McTighe, 1973); these positive findings could be explained on
the basis of a thermal mechanism because 2450 MHz is around the
resonant frequency for mice.
Changes in testicular morphology were observed by Varma &
Traboulay (1975) in mice exposed to 1.7 and 3.0 GHz microwaves at
10 mW/cm2 for 100 min, and at 50 mW/cm2 for 30-40 min. Both
Bereznitskaja (1968) and Polozitkov et al. (1961) reported that
chronic exposure of mice to 3 GHz at 10 mW/cm2 or even lower levels
resulted in a prolonged estrus cycle, partial sterility, and an
increased, early mortality of the offspring. However, other research
workers were unable to find any changes in the reproductive
performance of dogs exposed to 24 GHz microwaves at 24 mW/cm2 for 33
and 66 weeks (Deichman et al., 1963) and to 1.28 GHz at 20 mW/cm2
(Michaelson et al., 1971) or female rats and mice exposed to 3.1 GHz
at 8 mW/cm2 for prolonged periods of time and to 300 mW/cm2 for a
short period of time (Shore et al., 1977).
A comparative study of heating the testes of rats by microwaves
and in warm water was performed by Muraca et al. (1976). Radiation of
2.45 GHz was used at 80 mW/cm2 with the exposure time varied to
maintain the desired temperature within ± 0.5°C. Repeated treatments
during 5 consecutive days resulted in more damage in microwave-
irradiated animals. However, it was determined that if non-thermal
effects occurred, it appeared that microwave-induced heating was
necessary to produce damage.
The threshold value for testicular damage in dogs exposed to
2880 MHz microwaves at more than 10 mW/cm2 for unlimited periods was
studied by Ely et al. (1964). A temperature of 37°C was produced more
rapidly with exposure to higher power densities. This temperature was
determined as critical for damage based on a minimal demonstrable
histological change in the most sensitive animal from the test group.
The authors pointed out that the changes, including sterility, were
reversible.
In summary, microwave radiation can affect reproduction and
development. Both are particularly sensitive to thermal stress,
although specific effects that are not attributable to heating cannot
be excluded. Microwave exposure at power density levels causing
temperature increases results in testicular lesions, and particularly
affects spermatogenesis, in experimental animals. These lesions seem
to be readily reversible unless necrosis occurs. Baranski & Czerski
(1976) in their review of the subject concluded that no serious
effects should be expected at power density levels below 10 mW/cm2.
Substantial differences between thermal effects induced by microwaves
and by other methods of heating may be attributed to different spatial
distributions of internal heating and different rates of heating.
Developmental effects seem to be critically dependent on the time of
exposure to microwaves making it difficult to compare some of the
experimental data.
8. HEALTH EFFECTS IN MAN
The available data concerning the health effects of microwave
radiation in man are insufficient, although some surveys of the health
status of personnel occupationally exposed to microwaves have been
carried out. The main difficulty in the evaluation of such information
is the assessment of the relationship between exposure levels and
observed effects. As often happens in clinical work, it is difficult
to demonstrate a causal relationship between a disease and the
influence of environmental factors, at least in individual cases.
Large groups must be observed to obtain statistically significant
epidemiological data. The problem of adequate control groups is
controversial and hinges mostly on what is considered "adequate"
(Silverman, 1973; Czerki et al. 1974a; NAS/NRC, 1977).
In view of the lack of good instrumentation, especially of
personal dosimeters, the quantitation of exposure during work is
extremely difficult. This is particularly the case where personnel
move around in the course of their duties and are exposed to
stationary and non-stationary fields, and both near- and far-field
exposures. It is impossible to evaluate within reasonable limits the
exposure over a period of several years. Consequently, investigation
of the health status of personnel exposed occupationally to microwaves
necessitates the examination of large groups of workers exposed for
various periods, if any statistically valid results are to be
obtained.
Observations on the health status of personnel exposed to
microwaves in the USSR have been discussed in detail in monographs
edited by Petrov, ed. (1970) and Tjagin (1971).
8.1 Effects of Occupational Exposure
Prior to the establishment of safety standards, it had been
observed in some countries that occupational microwave exposure led to
the appearance of autonomic and central nervous system disturbances,
asthenic syndromes, and other chronic exposure effects (Gordon, 1966;
Marha et al., 1971; Dumanski et al., 1975; Serdjuk, 1977). The
pathogenesis of these syndromes is controversial, their existence has
been reported on a number of occasions but often without the level of
exposure. Another problem with earlier reports is that measurement
techniques were not properly developed at that time (For a detailed
discussion see Baranski & Czerski (1976) pp. 153-162). Subjective
complaints consisted of headaches, irritability, sleep disturbance,
weakness, decrease in sexual activity (libido), pains in the chest,
and general poorly defined feelings of in health. On physical
examination, tremor of fingers with extended arms, acrocyanosis,
hyperhydrosis, changes in dermographism, and hypotonia were reported
in the USSR (Gordon, 1966). Similar syndromes were reported in France
by Deroche (1971) and in Israel by Moscovici et al. (1974).
Examination of the circulatory function included determination of
the velocity of propagation of the pulse wave. Various coefficients
may be calculated and used for the evaluation of vascular tonus and
the state of the neurovegetative system. This method is widely used in
the USSR, but seldom elsewhere. Disturbances in the functioning of the
circulatory system are demonstrable using this method whereas, with
the exception of signs of bradycardia, no significant findings are
obtained using electro-, vecto-, and ballisto-cardiography.
Mechanocardiography demonstrated normal or increased systolic and
minute heart volume in individuals with hypotonia (Tjagin, 1971).
Gordon (1966) and her colleagues reported studies on
occupationally exposed workers who were divided into 3 groups
according to levels of exposure to microwave radiation:
(a) Periodic exposure at power densities from 0.1 to 10 mW/cm2
(and higher) of maintenance personnel and workers, who had been
employed in repair shops since 1953;
(b) Periodic exposure at power densities from 0.01 to 0.1 mW/cm2
of technical maintenance workers, some users of microwave devices, and
research workers, employed after 1960; and
(c) Systematic low-level exposure of personnel using various
microwave devices, mainly radar.
Functional changes in the nervous and cardiovascular systems were
reported in the first 2 groups. In the first group, a marked
disturbance in cardiac rhythm, expressed by variability or pronounced
bradycardia was reported. In the third group, similar effects were
observed but symptoms were less evident and easily reversed. Only
about 1000 individuals were observed over a period of 10 years and
some doubts exist regarding the exact exposure received by the
workers.
Clinical observations on the health status of 2 groups of workers
occupationally exposed to emissions from various types of radio
equipment were reported by Sadcikova (1974). The first group consisted
of 1000 workers exposed to RF radiation at a few mW/cm2, the second
group, of 180 workers who had been exposed at a few hundredths of a
mW/cm2 over short periods of time. A control group of 200 was
matched with respect to sex, age and character of work. The health
status of both exposed groups was reported to differ considerably from
that of the controls, with a higher incidence of changes in the
nervous and cardiovascular systems in the exposed groups.
In Poland (Siekierzynski, 1974; Czerski et al., 1974c;
Siekierzynski et al., 1974a, b), a selected group of 841 males, aged
20-40 years and occupationally exposed to microwaves at power
densities ranging from 0.2 to 6 mW/cm2, was studied. No relationship
was found between the level or length of occupational exposure and the
incidence of disorders or functional disturbances such as organic
lesions of the nervous system, changes in the translucent media of the
eye, primary disorders of the blood system, neoplastic diseases or
endocrine disorders, neurasthenic syndrome, disturbances of the
gastrointestinal tract, and cardiocirculatory disturbances with
abnormal ECG.
A 3-year epidemiological study aimed at determining health risks
from microwave exposure in US naval personnel was reported by
Robinette & Silverman (1977). Mortality, morbidity, reproductive
performance, and health of children were investigated in 20 000
occupationally exposed subjects and 20 000 controls. No significant
differences were found between the 2 groups.
Cases of whole body or partial body overexposure may occur among
personnel operating high-power equipment. Exposure of the head and
resultant injury to the brain have been reported (Servantie et al.,
1978). The person concerned may not realize that exposure is taking
place, if there is no sensation of heat. The symptoms may appear
later, and a syndrome of meningitis or symptoms similar to those of
heat stroke may develop.
It has been emphasized by many research workers, including
Silverman (1973), that the inadequacies and uncertainties of radiation
measurements and exposure data from existing, clinical studies, make
it impossible to determine if, and under what conditions, microwave
radiation can induce neural or behavioural changes in man.
Unfortunately, the same problem exists for other studies carried out
on human subjects exposed to microwaves, making it difficult to draw
conclusions on health status.
8.1.1 Effects on the eyes
Epidemiological surveys of lenticular effects in microwave workers
have been performed in Poland (Siekierzynski et al., 1974a, b;
Zydecki, 1974), Sweden, (Tengroth & Aurall, 1974) and the USA (Cleary
& Pasternack, 1966; Appleton & McCrossan, 1972; Shacklett et al.,
1975). No statistically significant increases in the number of
cataracts in personnel occupationally exposed to microwave radiation
were observed in any of the surveys. Tengroth & Aurell (1974)
indicated a statistically significant increase in lenticular defects
and retinal lesions in 68 workers in a Swedish factory, where
microwave equipment was tested. These authors were some of the first
to point out possible retinal lesions from exposure to microwaves.
However, survey data on the intensity of radiation were not provided
and the control group was not age-matched. Statistically significant
differences in lens opacities between exposed and control groups were
not found in any of the other surveys. In cases of confirmed
cataracts, there had been reported exposures at densities exceeding
100 mW/cm2; indeed, power densities as high as 1000 mW/cm2 were
cited.
8.1.2 Effects on reproduction and genetic effects
There is little information on the effects of microwave radiation
on male or female reproductive functions. Reports of sterility or
infertility from exposure to microwaves are questionable. No changes
in the fertility of radar workers were found by Barron & Baraff
(1958).
Marha et al. (1971) attributed decreased spermatogenesis, altered
sex ratio of births, menstrual pattern changes, congenital effects in
newborn babies, and decreased lactation to the occupational exposure
of mothers to RF radiation. According to their report, such effects
occurred at power densities exceeding 10 mW/cm2.
8.1.3 Cardiovascular effects
Functional damage to the cardiovascular system as manifested by
hypotonus, bradycardia, delayed auricular and ventricular
conductivity, and flattening of ECG waves, has been reported, by
several USSR clinicians, to result from chronic exposure of workers to
RF fields (Gordon, 1966, 1967; Tjagin, 1971; Baranski & Czerski,
1976). Decreases in blood pressure from exposure have also been
reported. Some authors in the USSR have indicated that the nature and
seriousness of cardiovascular reactions to prolonged exposure is
related to changes in the nervous system, and depends on the
characteristics of the individual. Some patients exhibited only minor
asthenic symptoms while others developed marked autonomic vascular
dysfunction.
8.2 Medical Exposure
Controlled follow-up studies of patients treated with microwave
and RF diathermy could yield important data on effects, at least for
partial body exposure. Such studies could not be found in the
available literature. However, cases of congenital malformations
ascribed to exposure to microwave or RF diathermy during early
pregnancy have been found in the literature by Marha et al. (1971).
9. RATIONALES FOR MICROWAVE AND RF RADIATION PROTECTION STANDARDS9.1 Principles
An important part of the rationale for standards should be the
definition of the population to be protected. Occupational health
standards are aimed at protecting healthy adults exposed under
controlled conditions, who are aware of the occupational risk and who
are likely to be subject to medical surveillance. General population
standards must be based on broader considerations, including health
status, special sensitivities, possible effects on the course of
various diseases, as well as limitations in adaptation to
environmental conditions and responses to any kind of stress in old
age. As many of these considerations involve insufficiently explored
interactions, standards for the general population must involve
adequate safety factors, including taking into account the possibility
of 24-h general population exposure compared with 8-h occupational
exposure.
A distinction should be made between exposure limits for workers
and equipment emission standards. The latter are based on safe
operational considerations, should be derived from exposure limits,
and they should not allow exposure above the adopted exposure limits.
The USA performance standard for microwave ovens (US Code of Federal
Regulations, 1970) may serve as an example. This standard limits the
emission of unintentional radiation (microwave leakage) to 1 mW/cm2
at a distance of 5 cm from the surface of the oven. Only a few
countries have formally adopted standards. Where standards have been
promulgated, the procedures of enforcement vary from regulations
enforceable by law to voluntary guidelines.
Existing radiation protection guides (RPG) or exposure standards
may be divided into 3 groups according to the exposure limits adopted
(Czerski, 1976).
The first group comprises standards and recommendations in which
microwave exposure of the order of tens of microwatts/cm2 (up to
100 µW/cm2) is allowed. The second group includes exposures of the
order of hundreds of microwatts/cm2 (1000 µW/cm2), and the third
group allows exposures of thousands of microwatts/cm2
(10 000 µW/cm2). This division does not correspond to any
classification of RPG or exposure limits on a national or regional
geographical basis. As exposure standards have been revised or
introduced they have recently tended towards group 2 (Repacholi,
1978).
9.2 Group 1 Standards
The first group is represented by the exposure standards of
Bulgaria (Bulgarian National Standard, 1979) and the USSR (Ministry of
Health of the USSR, 1970; USSR Standard for Occupational Exposure,
1976; USSR Standard for Public Exposure, 1978).
The original USSR occupational microwave exposure limits were
established in 1959 (Ministry of Health of the USSR, 1970). The
current standard (USSR Standard for Occupational Exposure, 1976)
reaffirmed the exposure limits for microwaves (300 MHz-300 GHz) and
introduced exposure limits for RF (60 kHz-300 MHz). A special standard
was introduced for public exposure in 1978 (USSR Standard for Public
Exposure, 1978). Detailed data on exposure limits are given in Table
18 at the end of section 9.5. The exposure limits representative for
this group of standards are those of the USSR occupational standard
(USSR Standard for Occupational Exposure, 1976): microwave radiation
(300 MHz-300 000 MHz) at working locations should not exceed 10
microwatt/cm2 (0.1 W/m2) for exposure during the whole working
day, 100 microwatt/cm2 (1 W/m2) for exposures of not more than 2 h
per working day, and 1000 microwatt/cm2 (10 W/m2) for exposures of
not more than 15-20 min per working day, providing that protective
goggles are used and that the radiation (exposure) does not exceed 10
microwatt/cm2 (0.1 W/m2) during the rest of the working day.
The principle of establishing exposure levels in standards,
according to Gordon (1966, 1970), is the avoidance of risks during
long-term (many years) occupational exposure.
In the USSR standard concerning general population exposure to
microwaves in the range of 300 MHz-300 GHz (Fig. 15), a value of
5 µW/cm2 has been adopted as the exposure limit over a 24-h period,
for inhabited areas. This standard covers radiation from scanning and
rotating antennae, which turn with a frequency below 0.5 Hz. The
irradiation time of a point in space should not exceed one tenth of
the scanning duty cycle, and the relation between the maximum energy
levels in comparable time intervals should not exceed 10.
The USSR occupational and public health safety standards are based
on the principle of complete prevention of health risks and therefore
include large safety factors.
9.3 Group 2 Standards
The second group of standards may be illustrated by those of
Czechoslovakia (Principal Hygienist of CSSR, 1965, 1970), the German
Democratic Republic Standard (GDR Standard TgL 32602/01, 1975), the
Polish regulations on microwave exposure limits (Council of Ministers,
1972) and RF exposure limits (Ministers of Labour, Wages and Social
Affairs and Health and Social Welfare, 1977), as well as the USA Bell
Telephone recommendations (Weiss & Mumford, 1961). The recently
introduced Canadian (National Health and Welfare, Canada, 1979) and
Swedish (IVA-Committee, 1976; Worker Protection Authority, 1976)
exposure standards and the Australian proposal (Cornelius & Vigilione,
1979) might also be placed in this group.
The Czechoslovak standard is discussed in detail by Marha et al.
(1971), who claim that "biological knowledge" was taken into account
in establishing the permissible exposure levels and a safety factor of
10 introduced. Ten microwatts/cm2 (0.1 W/m2) mean power density
was accepted as safe for long-term exposures to pulsed waves. For the
"demonstrably less risky" continuous wave exposure, 25
microwatts/cm2 was permitted. These rules were first adopted in 1965
and were revised in 1970 (Principal Hygienist of CSSR, 1965, 1970) in
order to incorporate a time-weighted averaging procedure.
RPG values based on an 8-h working day for occupational exposure
and over 24-h for the general population have also been introduced.
For occupational continuous wave exposure to microwaves, the exposure
limit may not exceed 25 microwatts/cm2. The permissible exposure
levels are calculated according to a formula from which a continuous
wave exposure to 1.6 mW/cm2 or pulsed wave exposures to
0.64 mW/cm2 during 1-h per working day are permissible. This is
considerably higher than the values accepted in the USSR. No advice is
given concerning exposure lasting for a few minutes. Details of
measuring methods and equipment are also given by Marha et al. (1971).
Continuous generation is defined as operation with a ratio of
on-to-off time greater than 0.1. The overall impression is that the
values accepted in Czechoslovakia for short periods of exposure (2 or
10 min), may be compared with those recommended by the American
National Standards Institute (ANSI, 1974, 1979).
Poland adopted the same exposure limits as those of the USSR in
1961. In 1963, additional information on interpretation was
introduced. Effective irradiation time was defined by the following
expression for far-field exposure to intermittent radiation from
scanning beams:
tef = (phi/360) tp
where tef = effective irradiation time (h)
tp = time of emission of microwaves
phi = effective beam width in degrees.
Special formulae were given for the near-field zone. Because of the
difficulties of solving all the doubts arising out of practical
situations, new exposure limits were subsequently proposed. These were
based on detailed discussions of findings in Czechoslovakia, the USSR
and the USA, radiation protection guides, standards, and rules, and
epidemiological analysis of the health status of personnel
professionally exposed to microwaves (Czerski & Piotrowski, 1972). The
new proposals were accepted and introduced in laws passed in Poland by
the Council of Ministers (1972) and the Minister of Health and Social
Welfare (1972) (Fig. 16). For the general population, the values of 10
and 100 µW/cm2 were adopted for continuous and intermittent
exposures, respectively. These values were taken as the upper limits
for a safe zone, in which occupation could be unrestricted. Three
other zones were defined, based on power density. For stationary
(continuous) fields, these were:
(a) safe zone -- the mean power density not to exceed 0.1 W/m2,
human exposure unrestricted;
(b) intermediate zone -- minimum value 0.1 W/m2, upper limit
2 W/m2, occupational exposure allowed during a whole working day
(normally 8 h, but, in principle, could be extended to 10 h);
(c) hazardous zone -- minimum value 2 W/m2, upper limit
100 W/m2, occupational exposure time per 24 h to be determined by
the formula:
t = 32/ p2
where t = exposure time (h) and p = mean power density (W/m2);
(d) dangerous zone -- mean power density in excess of 100 W/m2
(10 mW/cm2), human exposure forbidden.
For exposures to non-stationary fields, i.e., intermittent
exposure, the following values were adopted:
(a) safe zone -- mean power density not to exceed 1 W/m2
(0.1 mW/cm2);
(b) intermediate zone - minimum value 1 W/m2, upper limit
10 W/m2, occupational exposure allowed during a whole working day,
as defined earlier;
(c) hazardous zone -- minimum value 10 W/m2, upper limit
100 W/m2, the professional exposure time per 24 h to be determined
by the formula:
t = 800/ p2
where t = exposure time (h) and p = mean power density (W/m2);
(d) dangerous zone -- mean power density in excess of 100 W/m2
(10 mW/cm2), human exposure forbidden.
The Polish law (Council of Ministers, 1972) names the bodies to be
responsible for health surveillance, supervision of working
conditions, and the manner of carrying out the measurements (in
principle, every 3 years, and after changes in equipment or its
displacement). The main responsibility for decisions on admissibility
of working conditions rests with the sanitary epidemiological stations
of the Public Health Service. Newly designed equipment must be
evaluated by the Ministry of Health and Social Welfare, before
production and/or installation is allowed. For installation of
microwave equipment, permission is required from the sanitary
epidemiological station of the province.
These Polish regulations, in common with those of Czechoslovakia
and the USSR, have the following characteristics:
(a) Exposure limits determined separately for occupational and
general population exposures, medical examinations of microwave
workers limit occupationally exposed population to healthy adults
only;
(b) unified methods of measurement, measuring equipment, and
evaluation of results for health purposes;
(c) unified methods of medical examinations and evaluation of the
results obtained;
(d) determination of responsibility for compliance with RPG.
Sweden has introduced regulations for occupational exposure with
1 mW/cm2 as the normal limit for microwave exposure and allows
short-term excursions to a maximum of 25 mW/cm2. In the range
10-300 MHz, the limits are 5 mW/cm2 and 25 mW/cm2 (IVA Committee,
1976; Workers Protection Authority, 1976).
A new Canadian standard has now been published (National Health
and Welfare, Canada, 1979). This standard was developed following an
in-depth scientific evaluation of the literature (National Health and
Welfare, Canada, 1977, 1978) and was proposed (Repacholi, 1978) as a
draft so that it could be extensively reviewed. The final standard
applies to both occupational exposure and exposure of the general
population. Table 17 summarizes the exposure limits for whole or
partial body exposure to either continuous or modulated
electromagnetic radiation in the frequency range 10 MHz-300 GHz.
Higher occupational exposure is permitted for periods of less than
1 h. However, the maximum power density, averaged over a 1-min period
should not exceed 25 mW/cm2. Fig. 17 shows the permitted
occupational exposure in Canada.
Recently, the Australian Radiation Laboratory published a draft
proposal (Cornelius & Viglione, 1979) for exposure limits in the range
of 10 MHz-300 GHz. The values proposed are shown in Fig. 18 and,
according to the authors, are the result of a "worst case analysis".
Although, the rationale and some of the calculations presented in this
paper may be questioned on a formal basis, it is interesting to note
that the values for exposure limits agree well with those of the group
2 of standards.
Table 17. Canadian exposure limits for whole or partial
body exposure continuous or intermittent radiation
from 10 MHz-300 GHza
Group Frequency range Exposure limits
General 10 MHz--300 GHz 1 mW/cm2
population 60 V/m
0.16 A/m
Averaged over 1 min
Occupational 10 MHz--1 GHz 1 mW/cm2
60 V/m
0.16 A/m
Averaged over 1 h
1 GHz--300 GHz 5 mW/cm2
140 V/m
0.36 A/m
Averaged over 1 h
a From: National Health and Welfare, Canada (1979).
9.4 Group 3 Standards
The third group of standards may be illustrated by the 1966 US
Army regulations (Polmisans & Peczenik, 1966), The American National
Standards Institute Standard (ANSI, 1966) and recommendations of the
American Conference of Governmental Industrial Hygienists (ACGIH,
1971, 1979). Fig. 19 presents a comparison of these standards with the
Bell Telephone recommendations (Weiss & Mumford, 1961).
The US Army Standard is obviously intended as an occupational
safety RPG to microwaves and RF. Unlimited exposure is allowed at
levels below 10 mW/cm2 but exposure at power densities higher than
100 mW/cm2 is considered dangerous. Within the range of
10-100 mW/cm2, exposure is allowed for a limited time according to
the formula:
t = 6000/ Pd2
where t = exposure time (min) and
Pd = mean power density (mW/cm2).
A practical upper limit of 55 mW/cm2 is imposed, based on the
assumption that exposure of less than 2 min duration cannot be
properly regulated.
It is also recommended that, wherever possible, exposure levels
inside military installations should be reduced to a minimum.
The recommendations drawn up by the C-95.1 Committee of the
American National Standards Institute do not set an upper limit of
exposure (ANSI, 1966). However, this limit is defined indirectly by
the recommendation that the power density to which people may be
exposed should not exceed 10 mW/cm2 as averaged over any period of
0.1 h. No distinction between pulsed or continuous wave exposure is
made. These recommendations allow a 1-min exposure to 60 mW/cm2,
which may be repeated 10 times per hour. US Army recommendations allow
only a single 2-min exposure at 55 mW/cm2 during 1 h. On the other
hand, ANSI recommendations allow a 6-min exposure at only 10 mW/cm2,
while the US Army accepts 32 mW/cm2 for this period.
The ANSI C.95.1 Committee revised its standard in 1974 (ANSI 1974)
and is considering a draft proposal to reduce the permissible exposure
limits (ANSI, 1979). A comparison of the ANSI exposure limits in
successive standards can be found in Table 18 at the end of this
section.
The American Conference of Governmental Industrial Hygienists has
also made recommendations for the frequencies of 300 MHz-300 GHz
(ACGIH, 1971, 1979, 1980):
(a) For average power density levels up to, but not exceeding
10 mW/cm2, total exposure time should be limited to the 8-h working
day (continuous exposure).
(b) Exposure to higher average power density levels is permitted
for short periods of time. For example, exposure to 25 mW/cm2 is
permitted for 2.4 min during each 6-min period in an 8-h working day
(intermittent exposure);
(c) For average power density levels exceeding 25 mW/cm2, no
exposure is permissible (ceiling value).
(d) Under conditions of moderate to severe heat stress, the values
recommended may need to be reduced.
These Group 3 recommendations are based on human thermal balance
characteristics contained in the studies by Schwan & Piersol (Schwan,
1978; Schwan & Piersol, 1954, 1955) on the biophysical and
physiological aspects of the absorption of electromagnetic energy in
body tissues. These views (Schwan, 1976) can be presented briefly as
follows:
(a) the principal effects of microwaves consist of temperature
increases in the irradiated object;
(b) because of heat balance characteristics in man, indefinite
exposure to 10 mW/cm2 is possible, higher values may be accepted for
short-term exposure;
(c) the formation of cataracts or lenticular opacities cannot be
expected at power densities below 100 mW/cm2;
(d) biophysical considerations exclude the possibility of
microwave interaction with nerve cells;
(e) there is no evidence of untoward effects of microwave
radiation in man at power densities below 10 mW/cm2.
The US ANSI Committee is in the process of revising its standard
and it appears likely from review documents, which have been
circulated, that the 10 mW/cm2 value will be reduced to 1 mW/cm2
in the frequency range 30-300 MHz with increased levels on either side
of this frequency range (ANSI, 1979). This standard would then come in
Group 2.
9.5 RF Radiation Standards (100 kHz to 300 MHz)
The USA exposure limits covering the range 10 MHz-100 GHz and some
other national standards (e.g., the United Kingdom) include a part of
the RF range. Standards intended specifically for RF radiation (as
defined by international agreement), have been introduced only in
Czechoslovakia (Principal Hygienist of CSSR, 1965, 1970), the German
Democratic Republic (GDR Standard -- TGL 32602, 1973), Poland
(Ministers of Labour, Wages and Social Affairs and of Health and
Social Welfare, 1977), and in the USSR (USSR Standard for Occupational
Exposure GOST 12.1.00.76, 1976; USSR Standard for Public Exposure
SN-1823-78, 1978).
In the USSR occupational standard, values from 5 V/m to 50 V/m
have been adopted in the range of 60 kHz to 300 MHz (see Table 18).
The USSR values for inhabited areas (public health standards) are as
follows:
E field
in the range of 30--300 kHz 20 V/m
in the range of 0.3-- 3 MHz 10 V/m
in the range of 3-- 30 MHz 4 V/m
in the range of 30--300 MHz 2 V/m
In the Czechoslovak standard (Marha et al., 1971), the approach to
RF exposure is similar to that for microwave exposure. The permissible
exposure duration for the frequency range 30 kHz-30 MHz is calculated
from the formula:
E × t = 120
where E = peak electric field strength (V/m)
t = time (h)
For 24-h exposure, 5 V/m is considered safe. In the range of
30 MHz-300 MHz, the equivalent is 1 V/m.
Occupational exposure guide values are: 400 for 40 kHz-30 MHz and
80 for the range 30 MHz-300 MHz allowing 50 V/m and 10 V/m,
respectively, for an 8-h working day.
The Polish proposal uses the concept of 4 zones, i.e., safe,
intermediate, hazardous, and dangerous, and exposure limits presented
in Fig. 20 and 21 are of the same order of magnitude as those for
Czechoslovakia and the USSR.
Table 18 includes examples of microwave and RF exposure limits
adopted or proposed by various countries.
Table 18. Examples of microwave and RF exposure limits in various countries
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
Australia
Australian Draft proposal Frequency (f) MHz dependent 24 h both both Compare Fig. 18;
Radiation for both limit (L) mWh/cm2 for time a proposal for
Laboratory occupational integrated exposures average near-field
(Cornelius & and public over any 1-h period exposure limits
Viglione, exposure and 4 (L) mW/cm2 averaged is also included,
1979) over any 1-s period for peak pules
periods of less than 1 h exposure is
limited to
1 W/cm2.
10-30 MHz L = 5.4 - 0.365 f + 0.0064 f2
30-130 MHz L = 0.2
130-600 MHz L = 0.2 + 0.00128 (f + 130)
0.6-3 GHz L = 0.8 + 0.00029 (f + 600)
3-300 GHz L = k.5
Bulgaria
State Legal national Electric field strength V/m
(National) standards; 60 kHz-3 MHz 50 V/m working day - -
Committee for enforceable 3 MHz-30 MHz 20 V/m working day
standardization by law; 30 MHz-50 MHz 10 V/m working day
(1979) occupational 50 MHz-300 MHz 5 V/m working day
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
Magnetic field strength A/m working day
60 kHz-1.5 MHz 5 A/m working day
30 MHz-50 MHz 0.3 A/m working day
Power density W/m2 working day
300 MHz-300 GHz up to 0.1 working day both stationary
300 MHz-300 GHz 0.1-1 W/m2 no more than both stationary Up to 0.1 during
2 h the remainder
of the working
day.
300 MHz-300 GHz 1.0-10.0 W/m2 no more than both stationary Up to 0.1 during
20 min the remainder of
the working day-
protective goggles
required.
300 MHz-300 GHz up to 1.0 W/m2 working day both rotating
300 MHz-300 GHz 1.0-10.0 W/m2 no more than both rotating If the ambient
2 h temperature is
over 28 °C or
simultaneous
exposure to
X-rays occurs,
exposures over
1.0 W/m2 are
not allowed.
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
Canada
Canadian Voluntary, 10 MHz-100 GHz 10 mW/cm2 no limit cw both No longer applies,
Standards occupational as the 1979
Association national standard
(1966) is more
conservative.
1 mWh/cm2 0.1 h pulsed both
National National
Health & health and
Welfare occupational
(1979) safety
regulation,
enforceable by
law.
Occupational 10 MHz-1 GHz 1 mW/cm2 power density no limit both both See also Fig. 17.
60 V/m rms electric field averaged
strength over 1 h
0.16 A/m rms magnetic field
strength
1 GHz-300 GHz 5 mW/cm2 power density averaged over both both
140 V/m rms electric field 1 h
strength
0.36 A/m rms magnetic field
strength
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
10 MHz-300 GHz 25 mW/cm2 power density 1 min both both These values
300 V/m rms electric field cannot be exceeded
strength and constitute
0.8 A/m rms magnetic field "ceiling levels".
strength Some provisions
for "special"
cases, under
strictly
controlled
conditions were
added. 10 mW/cm2
cannot be exceeded
when averaged over
1-h period.
General 10 MHz-300 GHz 1 mW/cm2 power density no limit,
population 60 V/m rms electric field averaged
strength over 1 min
0.16 A/m rms magnetic field
strength
Czechoslovakia
Principal National
Hygienist health and
of the CSSR occupational
(1970) safety
regulation
enforceable
by law
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
Occupational 30 kHz--30 MHz Exposure limit (L) V/m Exposure - - Occupationally
calculated according to duration (t) exposed persons
formula: in hours are under
L × t (h) = 400, i.e., 50 V/m calculated obligatory medical
for 8 h according to surveillance
30 MHz--300 MHz L × t (h) = 80, i.e., 10 V/m the formula in (periodical
for 8 h the next column examinations as
to the left specified by law).
300 MHz--300 GHz Exposure limit (L) µW/cm2 Unified measuring
calculated according to methods imposed
formula L × t (h) = 200 i.e., and specified by
25 µW/cm2 for 8 h as above cw both the same
regulations. The
300 MHz--300 GHz L × t (h) = 80, i.e., 10 µW/ regulations dated
cm2 for 8 h as above pulsed both 1965 limited peak
pulse power
General 30 kHz--30 MHz Exposure limit (L) V/m as above -- -- (instantaneous
population calculated according to exposure) to 1 kW/
formula L × t (h) = 120, omitted in the
i.e., 5 V/m for 24 h revision dated
30 MHz--300 MHz Exposure limit (L) V/m as above - - 1970.
calculated according to
the formula L × t (h) = 24.
i.e., 1 V/m for 24 h
300 MHz--300 GHz Exposure limit (L) µW/cm2 as above cw both
calculated according to the
formula L × t (h) = 60.
i.e., 2.5 µW/cm2 for 24 h
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
300 MHz--300 GHz L × t (h) = 24, i.e., µW/ as above pulsed both
cm2 for 24 h
German Democratic Republic
National National 60 kHz--3 MHz 50 V/m electric field strength working day _ _ Supersedes a
Committee occupational 3 MHz--30 MHz 20 V/m electric field strength working day _ _ standard dated
for health 30 MHz--50 MHz 10 V/m electric field strength working day -- -- 1972, microwave
Standardization, standards, 50 MHz--300 MHz 5 V/m electric field strength working day -- -- exposure limits
Measurements enforceable 300 MHz--300 GHz 10 µW/cm2 power density up to 8 h both stationary did not change,
and Products by law 300 MHz--300 GHz 100 µW/cm2 power density up to 2 h both stationary RF exposure limits
Control (1975) 300 MHz--300 GHz 1000 µW/cm2 power density up to 20 min both stationary were introduced
by the new
version.
300 MHz--300 GHz 100 µW/cm2 power density up to 8 h both rotating
300 MHz--300 GHz 1000 µW/cm2 power density up to 2 h both rotating 1000 µW/cm2 is a
"ceiling level"
that cannot be
exceeded.
Poland
Council of National 300 MHz--300 GHz up to 0.1 W/m2 (safe zone) unlimited both stationary Supersedes a 1961
Ministers regulation, (implicit regulation
(1972) enforceable general public) establishing
by law essentially the
same exposure
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
300 MHz-300 GHz 0.1 W/m2-2 W/m2 working day both stationary limits, as those
(intermediate zone) in the USSR.
300 MHz-300 GHz 2 W/m-100 W/m232 hours both stationary Although an
(hazardous zone) P2 occupational
standard. It
300 MHz-300 GHz Exceeding 100 W/m2 human both stationary established a
(danger zone) occupancy "safe" zone,
prohibited within which
betide (ceiling human occupancy
level) is unrestricted.
300 MHz-300 GHz up to 1 W/m2 (safe zone) unlimited both rotating Only workers
(implicit (persons
general public) occupationally
exposed) having a
medical
certificate of
fitness and
300 MHz-300 GHz 1 W/m2-10 W/m2 working day both rotating subject to
(intermediate zone) periodic medical
examinations may
enter the
"intermediate"
300 MHz-300 GHz 10 W/m2-100 W/m2800 hours both rotating and "hazardous"
(hazardous zone) P2 zones. In this
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
300 MHz-300 GHz Exceeding 100 W/m2 human occupancy both rotating way an implicit
(danger zone) prohibited general population
(ceiling exposure limit has
level) been established.
A regulation
establishing
general public
and environmental
protection
exposure limits
for microwave, RF,
and ELF was
drafted and will
be adopted in
1980. Compare
Fig. 16 and
section 9.3.
Occupational
exposure durations
and definitions of
electromagnetic
fields, stationary
versus rotating
antennae,
determined by a
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
separate
regulation
(Minister of
Health
& Social Welfare,
1972). P = power
density W/m2.
The Ministers National 0.1 MHz-10 MHz 20 V/m rms electric field unlimited -- -- The same concept
of Labour, regulation strength (safe zone) (implicit of safe,
Wages and enforceable general intermediate,
Social Affairs by law population) hazardous, &
and for Health danger zones makes
and Social 20 V/m-70 V/m rms electric working day both _ the standard an
Welfare (1977) field strength (intermediate implicit general
zone) population one.
Within the 0.1-
10 MHz range, rms
70 V/m-1000 V/m rms electric 560 both rotating magnetic field
field strength (hazardous E strength values
zone) were given but,
as they exceed
Exceeding 1000 V/m rms human both rotating corresponding
electric field strength occupancy rms electric
(danger zone) prohibited field values, only
(ceiling level) these are used in
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
up to 7 V/m rms electric unlimited both rotating practice, as the
field strength (safe zone) (implicit limiting factor in
general permissible
population) exposure values.
Compare Fig.
21 and Fig. 22.
10 MHz-300 MHz 7 V/m-20 V/m rms electric working day E = ams electric
field strength (intermediate field strength.
20 V/m-300 V/m rms electric 3200 both rotating
field strength (hazardous E2
zone)
Exceeding 300 V/m rms human both rotating
electric field strength occupancy
(danger zone) prohibited
(ceiling level)
Sweden
Workers National 10-300 MHz 5 mW/cm2 8 h both rotating
Protection occupational 0.3-300 GHz 1 mW/cm2 8 h
Authority safety 0.3-300 GHz 1-25 mW/cm260 both rotating P = power density
(1976) regulation P mW/cm2.
10 MHz-300 GHz 25 mW/cm2 averaged both rotating Ceiling level.
over
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
USA
American Occupational 10 MHz-300 GHz 10 mW/cm2 no limit (for both both Under "moderate
National voluntary periods of environmental"
Standards consensus 0.1 h or more) conditions, people
Institute standard with circulatory
(ANSI) (recommendation) difficulties and
(1966) certain other
ailments are
1 mWh/cm2 during any both both more vulnerable.
0.1-h period Techniques end
instrumentation
ANSI (1974) Occupational 10 MHz-300 GHz 10 mW/cm2 power density no limit cw both for measurements
voluntary 200 V/m electric field strength are given in
consensus 0.5 A/m magnetic field ANSI-C95.3-1973
standard strength publication.
(recommendation) Prevention of
associated hazards
-- see Institute
of Makers of
Explosives (1971).
10 mW/cm2 power density 0.1 h pulsed both The US Department
1 mWh/cm2 energy density of Labour adopted
40 000 V2/m2 mean squared the ANSI 1968
electric field strength (E2) standard in its
0.25 A2/m2 mean squared proposed rules
magnetic field strength (A2) (Fed. Register,
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
Vol. 38, No.
166, para 1910,
345, p. 23046,
1973) and finally
adopted 10 mW/cm2
as maximum safe
exposure limit
for occupational
exposure (Fed.
Register, Vol. 40.
No. 59, point 12,
p. 13138, 1975).
This standard is
a recommendation
and the Dept of
Labour is
preparing a new
standard.
ANSI (1979) Draft proposal 0.3-3 MHz 100 mW/cm2 power density no limit, both both Mean squared
for voluntary 400 000 V2/m2-E2 averaged over electric field
consensus 2.5 A2/m2-H2 any 0.1-h strength (E2)
standard period and mean squared
(recommendation) magnetic field
strength (H2)
are applicable to
near-field
exposures.
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
3-30 MHz 900 mW/cm2 - power density averaged over both both f is the frequency
f2 any 0.1-h in MHz.
period
4000 × 900 V2/m2 - E2
f20.025 × 900 A2/m2 - H2
f2
30-300 MHz 1.0 mW/cm2 power density averaged over both both
4000 V2/m2-E2 any 0.1-h
0.025 A2/m2-H2 period
0.3-1.5 GHz f mW/cm2 averaged over both both
300 any 0.1-h
period
4000 × f V2/m2-E2
300
0.025 × f A2/m2-H2
300
1.5-300 GHz 5 mW/cm2 power density averaged over both both
20000 V2/m2-E2 any 0.1-h
0.125 A2/m2-H2 period
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
American Recommendation 10 MHz-100 GHz Same as ANSI (1974) same as ANSI both both A ceiling level of
Conference (1974) 25 mW/cm2 was
of added to ANSI
Governmental (1974)
Industrial recommendations.
Hygienists Note: A legally
(ACGIH) enforceable USA
(1979) standard is an
equipment emission
standard for
microwave ovens
(US Code of
Federal
Regulations,
1970).
USSR
National Occupational 60 kHz-3 MHz 50 V/m electric field strength working day both both Supersedes earlier
Standards national 3 MHz-30 MHz 20 V/m electric field strength regulations
Committee at standard, 30 MHz-50 MHz 10 V/m electric field strength and standards
the Council enforceable 50 MHz-300 MHz 5 V/m electric field strength (section 9.2)
of Ministers by law 60 kHz-1.5 MHz 5 A/m magnetic field strength working day both both without essential
of the USSR, 30 KHz-50 Mhz 0.3 A/m magnetic field strength working day both both changes.
1976 (USSR 300 MHz-300 GHz up to 0.1 W/m2 power density working day both stationary
Standard for 0.1 to 1.0 W/m2 power density up to 2 h both stationary During the
Occupational per day remainder of the
Exposure, working day up to
1976) 0.1 W/m2.
Table 18 (Cont'd)
Country,
agency, or Type of Exposure cw/ Antenna
organization, standard Frequency Exposure limit duration pulsed stationary/ Remarks
date rotating
300 MHz-300 GHz 1.0 to 10 W/m2 power density up to 20 min both stationary During the
per day remainder of the
working day up to
0.1 W/m2.
protective goggles
are required.
300 MHz-300 GHz up to 1.0 W/m2 power density during the both rotating
working day
1.0 to 10 W/m2 power density up to 2 h both rotating During the
per day remainder of the
working day up to
1.0 W/m2.
Ministry of Public health see section 9.2
Health regulation
Protection (USSR enforceable
Public Health by law
Standard,
1978) (1978)
10. SAFETY PROCEDURES FOR OCCUPATIONALLY EXPOSED PERSONNEL
Broadcasting, radio, radar, industrial heating, and medical
equipment are essentially the same in all countries. Thus, the field
strengths associated with different types of equipment will be similar
and typical examples are shown in Fig. 22. In many instances, much
higher field strengths may be encountered. Where personnel may be
exposed to potentially high field strengths, they should receive
appropriate training and be made aware of possible risks to health
from the improper use of the equipment. This is especially important
where the operation of equipment does not necessitate professional
training or skills, e.g., plastic sealers. It is likely that service
work or repairs will entail greater risk than operation since
protective devices such as screens and interlocks would, in many
cases, have to be rendered inoperable to carry out the servicing or
repairs. Furthermore, service or repair personnel would generally be
closer to the microwave/RF source.
Reviews of safety procedures can be found in Mumford (1961), ANSI
(1973), Minin (1974), Krylov & Jucenkova (1979), and National Health &
Welfare, Canada (1979) (Safety Code 6).
10.1 Procedures for Reducing Occupational Exposure
The basic methods of controlling and limiting microwave/RF
exposure, in order of desirability are: (a) engineering -- safe design
and construction; (b) siting; (c) administrative; and (d) personnel
protection.
All unnecessary emissions should be minimized at the source,
preferably by containment or otherwise effective screening. This
approach is clearly impractical as far as the antenna system of
deliberate emitters is concerned. In this case, siting can be very
important in keeping both the number of people, who may be exposed,
and the levels of exposure as low as possible. The same considerations
apply where emission is unintentional but some leakage is unavoidable.
Where people can be exposed to potentially hazardous levels,
access to such areas should be controlled and restricted to persons
who are trained and are aware of any risks. The use of special warning
signs described in the Health and Welfare Canada Safety Code 6
(National Health and Welfare, Canada, 1979) would be particularly
useful. Time spent in the area should be kept as short as possible
and, wherever practicable, microwave/RF power levels should be kept as
low as readily achievable.
The use of protective clothing is not generally recommended as it
may initiate other hazards to the wearer, e.g., RF burns.
For more detailed information, the reader is referred to the
Health and Welfare Canada Safety Code 6 (National Health and Welfare,
Canada, 1979).
11. ASSESSMENT OF DATA ON BIOLOGICAL EFFECTS AND RECOMMENDED EXPOSURE
LIMITS
Major difficulties exist in assessing the potential health hazards
to man of exposure to microwave and RF radiation, because of the
highly complex relationship between the exposure conditions and the
energy absorbed. The absorbed dose and rate of energy absorption
depend critically on such variables as frequency, power density, field
polarization, the size and shape of the exposed subject, and
environmental factors. Many of the experiments contain insufficient
information on the dosimetry, thus, difficulties arise in the exact
interpretation of results.
Experimental results indicate that most reported effects can be
explained on the basis of microwave-induced, nonuniform heating.
However, other investigations which have been carried out to evaluate
the mechanisms involved, e.g., comparison of effects induced by
microwaves with those produced by a water bath, indicate that
nonthermal mechanisms may be involved. Further thorough studies of
these nonthermal mechanisms are necessary, as their contribution to
the understanding of microwave effects may be of great importance.
Since most biological effects have been reported as phenomena,
little information exists on quantitated dose-effect relationships.
Studies on dose-effect threshold levels and their frequency dependence
are badly needed in most areas. Because of the lack of such data,
recommendations for exposure limits can only be made on the best
available interpretations of the literature. Such interpretations also
require an assessment of whether effects reported as phenomena truly
present a hazard to health. Many effects are transient or easily
reversible while others may cause permanent damage.
From the summaries in sections 7 and 8, the following
recommendations can be made:
(a) Effects have been reported at power densities too low to produce
biologically significant heating.
(b) The occupationally-exposed population consists of healthy adults
exposed under controlled conditions, who are aware of the occupational
risk. The exposure of this population should be monitored.
It is possible to indicate exposure limits from available
information on biological effects, health effects, and risk
evaluation. For workers, whole or partial body exposure to continuous
or pulsed microwaves or Rf radiation having average power densities
within the range 0.1-1 mW/cm2 includes a high enough safety factor
to allow continuous exposure to microwaves/RF from any part of the
frequency range, over the whole working day. Higher exposure may be
permissible over part of the frequency range and for intermittent or
occasional exposure. Special considerations may be indicated in the
case of pregnant women.
(c) The general population includes persons of different ages
(infants, small children, young adults and senior citizens) and
different states of health, including pregnant women. The possible
greater susceptibility of the developing fetus to microwave/RF
exposure may deserve special consideration. Exposure of the general
population should be kept as low as possible and limits should
generally be lower than those for occupational exposure.
In view of the fact that data are still required to clarify
interaction mechanisms and determine threshold levels for effects, it
is recommended that microwave and RF exposure of
occupationally-exposed workers and the general population should be
kept as low as readily achievable.
More precise exposure limits over the frequency range 100 kHz -
300 GHz for both occupational and general population exposure to
microwaves/RF will be recommended in follow-up documents.
REFERENCES
ACGIH (1971) Threshold limit values of physical factors with intended changes adopted by ACGIH for 1971, Cincinnati, OH, American
Conference of Governmental Industrial Hygienists.
ACGIH (1979) Threshold limit values documentation; Microwaves.
Cincinnati, OH, American Conference of Governmental Industrial
Hygienists, pp. 500-501.
ACGIH (1980) Threshold limit values for chemical substances and
physical agents in the workroom environment with intended changes
for 1980, Cincinnati, OH, American Conference of Governmental
Industrial Hygienists, p. 81.
ADDINGTON, C., FISHER, F., NEUBAUER, R., OSBORNE, C., SARKEES, Y., &
SWARTZ, G. (1958) Thermal effects of 200 megacycles (cw)
irradiation as related to shape location and orientation in the
field. In: Proceedings of the 2nd Tri-Service Conference on the Biological Effects of Microwave Energy, Charlottesville (ASTIA
Document No. AD 131477).
ADEY, W. R. (1975) Introduction: Effects of electromagnetic radiation
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GLOSSARY
Wherever possible, this glossary gives terms and definitions
standardized by the International Electrotechnical Commission in the
International Electrotechnical Vocabulary (IEV) or by the
International Organization for Standardization (ISO). In such cases,
the IEV number, or the number of the ISO standard in which the
definition appears, is given in parentheses. The great majority of the
terms and definitions are those of the IEV, and the help of Mr C. J.
Stanford, General Secretary, International Electrotechnical
Commission, in compiling the necessary information is gratefully
acknowledged.
An annex at the end of the glossary includes a number of
additional terms that have not been standardized.
absorption In radio wave propagation, attenuation of a radio wave
due to its energy being dissipated, i.e., converted into another
form such as heat (IEV 60-20-105).
absorption cross-section; effective area Of an [antenna], oriented
for maximum power absorption unless otherwise stated, an area
determined by dividing the maximum power absorbed from a plane
wave by the incident power flux density, the load being matched to
the [antenna] (IEV 60-32-035).
admittance The current flowing in a circuit divided by the terminal
voltage; the reciprocal of the impedance (IEV 05-40-035).
antenna That part of a radio system which is designed to radiate
electromagnetic waves into free space [or to receive them]. This
does not include the transmission lines or waveguide to the
radiator (IEV 60-30-005).
antenna directivity See directivity antenna gain See power gain of an antenna antenna, dipole See dipole antenna, horn See horn antenna, isotropic See isotropic radiator antenna pattern See radiation pattern antenna scanning See scanning attenuation The progressive diminution in space of certain
quantities characteristic of a propagation phenomenon
(IEV 05-03-115).
attenuation coefficient The real part of the propagation
coefficient. Synonym: attenuation constant (deprecated)
(IEV 55-05-255).
bel; decibel Transmission units giving the ratio of two powers. The
number of bels is equal to the logarithm to the base ten of the
power ratio. The decibel is equal to one-tenth of a bel
(IEV 55-05-120).
coaxial pair Two conductors, one being a wire or tube coaxially
surrounded by the other which is in the form of a tube
(IEV 55-30-45). A cable consisting principally of one or more
coaxial pairs is termed a coaxial cable (IEV 55-30-50).
conductance The reciprocal of resistance (IEV 05-20-170). Symbol:
G. Unit: siemens (S).
conductivity The scalar or matrix quantity whose product by the
electric field strength is the conduction current density
(IEV 121-02-1). It is the reciprocal of resistivity.
continuous wave A wave whose successive oscillations are, under
steady-state conditions, identical.
cross section A measure of the probability of a specified
interaction between an incident radiation and a target particle or
system of particles. It is the reaction rate per target particle
for a specified process divided by the flux density of the
incident radiation (microscopic cross section) (IEV 26-05-605).
current density A vector of which the integral over a given surface
is equal to the current flowing through the surface. The mean
density in a linear conductor is equal to the current divided by
the cross-sectional area of the conductor (IEV 05-20-045).
cycle The complete range of states or values through which a
phenomenon or periodic function passes before repeating itself
identically (IEV 05-02-050).
decibel See bel dielectric constant See permittivity dielectric (material) A material in which all of the energy required
to establish an electric field in the material is recoverable when
the field or impressed voltage is removed. A perfect dielectric
has zero conductivity and all absorption phenomena are absent. A
complete vacuum is the only known perfect dielectric.
dielectric saturation Response of a dielectric in the limit of high
electric field strengths, leading to a decrease of the real part
of the permittivity with increasing field strength.
dipole A centre-fed open [antenna] excited in such a way that the
standing wave of current is symmetrical about the mid point of the
[antenna] (IEV 60-34-005).
directivity That property of an [antenna] by virtue of which it
radiates more strongly in some directions than in others
(IEV 60-32-130).
displacement See electric flux density dissipation factor The reciprocal of the Q-factor (IEV 55-05-285).
See Q factor. duty factor The ratio of (1) the sum of pulse durations to (2) a
stated averaging time. For repetitive phenomena, the averaging
time is the pulse repetition period (IEV 531-18-15).
duty ratio The ratio, for a given time interval, of the on-load
duration to the total time (IEV 151-4-13).
effective area See absorption cross-section effective radiated power in a given direction The power supplied to
the [antenna] multiplied by the gain of the [antenna] in that
direction relative to a half-wave dipole (IEV 60-32-095).
electric charge; quantity of electricity Integral of electric
current over time (ISO 31/V). Symbol: Q. Unit: coulomb (C).
electric field strength A vector the value of which equals the force
exerted on a quantity of electricity divided by this quantity and
the direction of which is that of the force (IEV 05-15-45).
electric flux Across a surface element, the scalar product of the
surface element and the electric flux density (ISO 31/V).
electric flux density A vector quantity whose divergence equals the
electric volume charge density. Note: In vacuo, it is at all
points equal to the product of the electric field strength and the
electric constant (IEV 121-01-21). Symbol: D. Obsolete synonym:
displacement.
electric susceptibility The scalar or matrix quantity whose product
by the electric field strength is the electric polarization
(IEV 121-02-09).
electromagnetic energy The energy stored in an electromagnetic field
(IEV 121-01-39).
electromagnetic wave A wave characterized by variations of the
electric and magnetic fields (IEV 121-01-38).
electrostatic field That portion of the total electromagnetic field
produced by a current-carrying conductor or charge distribution,
the energy of which returns to the conductor when the current
ceases or the charge distribution goes to zero.
energy density See radiant energy density far zone See radiation zone ferromagnetic material A material in which the predominant magnetic
phenomenon is ferromagnetism. Note: The atoms or ions have
magnetic moments which, over certain regions (domains), are
aligned approximately in the same direction even in the absence of
an externally applied magnetic field. When such a field is
applied, the resultant moments of the domains tend to align so
that the material exhibits considerable permeability. The degree
of alignment within a domain decreases with increasing temperature
(IEV 901-01-29).
ferromagnetism A phenomenon by which the magnetic moments of
neighbouring atoms are aligned approximately in the same direction
due to mutual interaction (IEV 901-01-28).
field 1. In a qualitative sense, a region of space in which
certain phenomena occur. 2. In a quantitative sense, a scalar or
vector quantity the knowledge of which allows the effects of the
field to be evaluated (IEV 05-01-040).
field strength In radio wave propagation, the magnitude of a
component of specified polarization of the electric or magnetic
field. The term normally refers to the root-mean-square value of
the electric field (IEV 60-20-070).
flux See electric flux, magnetic flux flux density See electric flux density, magnetic flux density Fraunhofer region Of a transmitting [antenna] system, the region
which is sufficiently remote from the [antenna] system for the
wavelets arriving from the various parts of the system to be
considered to follow parallel paths (IEV 60-32-60).
free space An ideal, perfectly homogeneous medium possessing a
dielectric constant of unity and in which there is nothing to
reflect, refract, or absorb energy. A perfect vacuum possesses
these qualities.
frequency The reciprocal of period, q.v.
Fresnel region Of a transmitting [antenna] system, the region near
the [antenna] system where the wavelets arriving from the various
parts of the system cannot be considered to follow parallel paths
(IEV 60-32-065).
gain The increase in power between two points 1 and 2 at which the
power is respectively P 1 and P 2, expressed by the ratio
P 2/ P 1 in transmission units (IEV 55-05-185).
H field See table
horn An elementary [antenna] consisting of a waveguide in which one
or more transverse dimensions increase towards the open end
(IEV 60-36-055).
impedance The complex representation of potential difference divided
by the complex representation of current (ISO 31/V).
impedance characteristic Of a uniform transmission line, the
impedance with which one end of the line must be terminated in
order that the impedance presented at the other end shall have the
same value as the terminating impedance. Note: The term is
occasionally applied to a symmetrical two-terminal-pair network to
denote the common value assumed by the two image impedances and
the two iterative impedances (IEV 55-20-155).
impedance, wave (at a given frequency) The ratio of the complex
number (vector) representing the transverse electric field at a
point, to that representing the transverse magnetic field at that
point. The sign is so chosen that the real part is positive
(IEV 62-05-095).
induction field That part of the field of an [antenna] which is
associated with a pulsation of energy to and fro between the
[antenna] and the medium. Note: The induction field extends
theoretically over the whole of space, but is negligible compared
with the radiation field except in the neighbourhood of the
[antenna] (IEV 60-32-045).
induction zone; near zone The region surrounding a transmitting
[antenna] in which there is a significant pulsation of energy to
and fro between the [antenna] and the medium. Note: The magnetic
field strength (multiplied by the impedance of space) and the
electric field strength are inequal and, at distances less than
one tenth of a wavelength from an [antenna], vary inversely as the
square or cube or the distance, if the [antenna] is small compared
with this distance (IEV 60-32-055).
insertion loss The loss due to the insertion of a transducer between
two impedances ZE (generator) and ZR (load) is the expression
in transmission units of the ratio P 1/ P 2 where P 1 is
the apparent power received by the load ZR before the insertion
of the said transducer, and P 2 is the apparent power received
by the load ZR after the insertion of the said transducer
(IEV 55-05-160). Unit: decibel (dB).
irradiation, partial body Exposure of only part of the body to
incident electromagnetic energy.
irradiation, whole body Exposure of the entire body to incident
electromagnetic energy.
isotropic Having the same properties in all directions.
isotropic radiator An [antenna] which radiates uniformly in all
directions. This is a hypothetical concept used as a standard in
connection with the gain function (IEV 60-32-110).
joule The work done when the point of application of 1 ... unit of
force [newton] moves a distance of 1 metre in the direction of the
force (Comité international des Poids et Mesures, 1946).
magnetic field strength An axial vector quantity which, together
with magnetic induction, specifies a magnetic field at any point
in space. It can be detected by a small magnetised needle, freely
suspended, which sets itself in the direction of the field. The
free suspension of the magnetised needle assumes, however, that
the medium is fluid or that a small gap is provided of such a
shape and in such a direction that free movement is possible. As
long as the induction is solenoidal, the magnetic field is
irrotational outside the spaces in which the current density is
not zero, so that it derives a potential (non-uniform) therefrom.
On the other hand, in the interior of currents, its curl, in
the rationalised system, is equal to the vector current density,
including the displacement current.
The direction of the field is represented at every point by
the axis of a small elongated solenoid, its intensity and
direction being such that it counterbalances all magnetic effects
in its interior, whilst the field intensity is equal to the linear
current density of the solenoid (IEV 05-25-020). Symbol: H.
Unit: ampere per metre (A/m).
magnetic flux The area integral of the magnetic flux density
(IEV 901-01-04). Symbol: PHI. Unit: weber (Wb).
magnetic flux density A solenoidal axial vector quantity which at
any point defines the magnetic field at that point. Its value is
such that the force exerted on an electric charge at that point
moving at a given velocity is equal to the charge multiplied by
the vector product of the velocity and the magnetic flux density
(IEV 901-01-03). Symbol: B. Unit: tesla (T).
microwaves Electromagnetic waves of sufficiently short wavelength
that practical use can be made of waveguide and associated cavity
techniques in their transmission and reception (IEV 60-02-025).
( Note: for the purposes of the foregoing document the term is
taken to signify waves having an approximate frequency range of
0.3-300 GHz).
near zone See induction zone peak envelope power Of a radio transmitter, the average power
supplied to the [antenna] transmission line or specified
artificial load by a transmitter during one radio frequency cycle
at the highest crest of the modulation envelope, taken under
conditions of normal operation (IEV 60-42-260).
peak pulse amplitude See pulse amplitude peak pulse output power The maximum value of output power during a
stated time interval, spikes excluded (IEV 531-41-17).
period The minimum interval of the independent variable after which
the same characteristics of a periodic phenomenon recur
(IEV 05-02-40). Symbol: T. Unit: second (s).
permeability The scalar or matrix quantity whose product by the
magnetic field strength is the magnetic flux density. Note: For
isotropic media, the permeability is a scalar; for anisotropic
media, a matrix (IEV 121-01-37). Synonym: absolute permeability.
If the permeability of a material or medium is divided by the
permeability of cacuum (magnetic constant)m the result is termed
relative permeability. Symbol: µ. Unit: henry per metre (H/m).
permittivity; dielectric constant A constant giving the influence of
an isotropic medium on the forces of attraction or repulsion
between electrified bodies (IEV 05-15-120). Symbol: E. Unit:
Farad per metre (F/m).
permittivity, relative The ratio of the permittivity of a dielectric
to that of a vacuum (IEV 05-15-140). Symbol: Er. phase Of a periodic phenomenon, the fraction of a period through
which the time has advanced relative to an arbitrary time origin.
phase change coefficient The imaginary part of the propagation
coefficient. Note: This coefficient determines the change of
phase of the voltages or currents (IEV 55-05-260). Deprecated
synonyms: phase constant, wavelength constant. Symbol: ß.
Unit: radian per metre (rad/m).
polarization A vector quantity representing the state of dielectric
polarization of a medium, and defined at each point of the medium
by the dipole moment of the volume element surrounding that point,
divided by the volume of that element (IEV 05-15-115).
polarization, plane of In a linearly polarized wave, the fixed plane
parallel to the direction of polarization and the direction of
propagation. Note: In optics the plane of polarization is normal
to the plane defined above (IEV 60-20-010).
potential, electric For electrostatic fields, a scalar quantity, the
gradient of which, with reversed sign, is equal to the electric
field strength (ISO 31/V; also IEV 05-15-050).
power 1. Mean power, work (or energy) divided by the time in which
this work (or energy) was produced or absorbed. In periodic
phenomena, the average power during a period is generally taken.
2. Instantaneous power, the limit of the average power when the
interval of time considered becomes infinitely small
(IEV 05-04-025). Symbol: P. Unit: watt (W).
power flux density; field intensity In radio wave propagation, the
power crossing unit area normal to the direction of wave
propagation (IEV 60-20-075). Symbol: W. Unit: watts per square
metre (W/m2).
power gain The ratio, usually expressed in decibels, of (1) the
output power of an [amplifying device] operated under stated
conditions to (2) the driving power (IEV 531-17-26). Symbol: G. power gain of an [antenna] (in a given direction) The ratio, usually
expressed in decibels, of the power that would have to be supplied
to a reference [antenna] to the power supplied to the [antenna]
being considered, so that they produce the same field strength at
the same distance in the same direction; unless otherwise
specified, the gain is for the direction of maximum radiation; in
each case the reference [antenna] and its direction of radiation
must be specified, for example: half-wave loss-free dipole (the
specified direction being in the equatorial plane), an isotropic
radiator in space (IEV 60-32-115). Symbol: G. Unit: decibel
(dB).
Poynting vector A vector, the flux of which through any surface
represents the instantaneous electromagnetic power transmitted
through this surface (IEV 05-03-85). Synonym: power flux density.
propagation constant A complex constant characterizing the
attenuation and phase change per unit of length of the current or
voltages which are propagated along a uniform line supposed to be
infinitely long (IEV 05-03-150). Symbol: alpha. pulse amplitude The peak value of a pulse (IEVV 55-35-100).
pulse duration The interval of time between the first and last
instant at which the instantaneous value of a pulse (or of its
envelope if a carrier frequency pulse is concerned) reaches a
specified fraction of the peak amplitude (IEV 55-35-105).
pulse output power The ratio of (1) the average output power to (2)
the pulse duty factor (IEV 531-41-14).
pulse repetition rate The average number of pulses in unit time
during a specified period (IEV 55-35-125).
Q A measure of the efficiency of a reactive circuit (especially an
oscillating circuit) or a component thereof. Its precise
definition depends on the nature of the circuit; for an
oscillating system without lumped L or C it is equal to 2pi
times the average energy stored in the field divided by the energy
dissipated during one half cycle. Synonyms: Q factor, quality
factor.
radar The use or radio waves, reflected or automatically
retransmitted, to gain information concerning a distant object. The measurement of range is usually included (IEV 60-72-005). radar scan See scanning radiant flux (surface) density Quotient of the radiant flux at an
element of the surface containing the point, by the area of that
element (IEV 45-05-155). Symbol: E. When this quantity relates
to radiation incident on a surface, it is termed irradiance;
when it relates to radiation emitted from a surface, it is termed
radiant exitance Symbol: M (deprecated synonym: radiant
emittance) (IEV 45-05-160/170). Unit: watts per square metre
(W/m2).
radiation field That part of the field of an [antenna] which is
associated with an outward flow of energy (IEV 60-32-040).
radiation zone; far zone The region sufficiently remote from a
transmitting [antenna] for the energy in the wave to be considered
as outward flowing. Note: In free space, the magnetic field
strength (multiplied by the impedance of space) and the electric
field strength are equal in this region and, beyond the Fresnel
region, vary inversely with distance from the [antenna]. The inner
boundary of the radiation zone can be taken as one wavelength from
the [antenna] if the [antenna] is small compared with this
distance (IEV 60-32-050).
radiant intensity For a source in a given direction, the radiant
power leaving the source, or an element of the source, in an
element of solid angle containing the given direction, divided by
that element of solid angle (ISO 31/VI). Symbol: I. Unit: watt
per steradian (W/sr). With reference to antennas, this quantity is
also called radiated power per unit solid angle in a given direction (IEV 60-32-090).
radiation pattern; radiation diagram; directivity pattern A diagram
relating power flux density (or field strength) to direction
relative to the [antenna] at a constant large distance from the
[antenna]. Note: Such diagrams usually refer to planes or the
surface of a cone containing the [antenna] and are usually
normalized to the maximum value of the power flux density or field
strength (IEV 60-32-135).
radio frequency Any frequency at which electromagnetic radiation is
useful for telecommunication (IEV 55-05-060). (See Annex).
reactance Imaginary part of impedance (ISO 31/V). Symbol: X.
Unit: ohm (OMEGA).
reflected wave A wave, produced by an incident wave, which returns
in the opposite direction to the incident wave after reflection at
the point of transition (IEV 25-50-065).
reflection coefficient; return current coefficient The complex ratio
of reflected signal current to incident signal current at the
termination (IEV 55-20-180). Symbol: G. refractive index The ratio of the velocity of electromagnetic
radiation in vacuo to the phase velocity of electromagnetic
radiation of a specified frequency in a medium (ISO 31/VI).
Symbol: eta. scanning Of a radar [antenna], systematic variation of the beam
direction for search or angle tracking (IEV 60-72-095). The term
is also applied to periodic motion of a radiocommunication
antenna.
scattering The process by which the propagation of electromagnetic
waves is modified by one or more discontinuities in the medium
which have lengths of the order of the wave length (IEV
60-20-120); a process in which a change in direction or energy of
an incident particle or incident radiation is caused by a
collision with a particle or a system of particles (ISO 921). The
extent to which the intensity of radiation is decreased in this
manner is measured in terms of the attenuation coefficient (scattering). scattering cross section The cross section for the scattering
process (IEV 26-05-650). See cross section; scattering. shield A mechanical barrier or enclosure provided for protection
(IEV 151-01-18). The term is modified in accordance with the type
of protection afforded; e.g., a magnetic shield is a shield
designed to afford protection against magnetic fields.
standing wave A state of vibration in which the oscillatory
phenomena at all points are governed by the same time function,
with the exception of a numerical factor, varying from one point
to another (IEV 05-03-065).
standing-wave ratio The ratio of the maximum to the minimum
amplitude of the current, voltage or field, measured respectively
at an adjacent node and antinode in a line or waveguide carrying a
standing wave (IEV 60-32-235). Symbol: (S). thermograph A term applied to a variety of instruments for measuring
and recording temperature, especially (1) the heat radiated by the
human body and (2) atmospheric temperature. The record produced by
such an instrument is termed a thermogram and the technique is
termed thermography. Note: None of these terms should be used in
the context of thermal analysis, where they are deprecated.
time constant On an exponentially varying quantity, time after which
the quantity would reach its limit if it maintained its initial
rate of variation. If a quantity is a function of time given by
F(t) = A + Be -t/tau then tau is the time constant
(ISO 31/II).
transmission factor Ratio of the transmitted radiant ... flux to the
incident flux (IEV 45-20-085).
transmission loss Over a given transmission path and for a given
frequency, the amount, expressed in decibels, by which the
available power at the input to a receiver is less than that
available from the output stage of a transmitter (IEV 60-20-100).
wave A modification of the physical state of a medium which is
propagated as a result of a local disturbance (IEV 05-03-005).
wave, diffracted A wave caused by the scattering of an incident wave
upon an obstacle (IEV 101-05-15).
waveguide A system for the transmission of electromagnetic energy by
a wave not of TEM type. It may, for example, consist of a metal
tube, a dielectric rod or tube, or a single wire (IEV 62-10-005).
wave incident A travelling wave before it reaches a transition point
(IEV 25-50-055).
wavelength The distance between two successive points of a periodic
wave in the direction of propagation, in which the oscillation has
the same phase (IEV 05-03-030). Symbol: lambda. Unit: metre (m).
wave, plane A wave such that the corresponding physical quantities
are uniform in any plane perpendicular to a fixed direction
(IEV 05-03-010).
wave, transmitted A wave (or waves) produced by an incident wave
which continue(s) beyond the transition point (IEV 25-50-060).
wave, transverse A wave characterised by a vector at right angles to
the direction of propagation (IEV 05-03-070).
ANNEX
The terms and explanations included in this annex are for the
purposes of this publication only, and are not necessarily valid for
any other purpose.
athermal effect An effect in a living organism that occurs
predominantly as a result of some phenomenon other than a local or
whole body rise in temperature.
depth of penetration For a plane-wave electromagnetic field incident
on the boundary of a lossy medium, the depth of penetration of the
wave is taken to be that depth at which the field strength of the
wave has been reduced to 1/ e or approximately 37% of its
original value.
exposure, high level At the Warsaw symposium it was agreed that, in
the microwave range, "high-level exposure"covers exposure to power
flux densities exceeding 10 mW/cm2. There is no agreement as to
the meaning of the term when applied to radiation in the RF range.
exposure, intermittent This term refers to alternating periods of
exposure and absence of exposure varying from a few seconds to
several hours. If exposure lasting a few minutes to a few hours
alternates with periods of absence of exposure lasting 18-24 hours
(exposure repeated on successive days), "repeated exposure" might
be a more appropriate term.
exposure, long-term This term indicates exposure during a major part
of the lifetime of the animal involved; it may, therefore, vary
from a few weeks to many years in duration.
exposure, low-level At the Warsaw symposium it was agreed that, in
the microwave range, "low-level exposure" covers exposure to power
flux densities up to 1 mW/cm2. There is no agreement on the
meaning of the term when applied to radiation in the RF range.
exposure, medium-level At the Warsaw symposium it was agreed that,
in the microwave range, "medium-level exposure" covers exposure to
power flux densities of 1-10 mW/cm2. There is no agreement on
the meaning of the term when applied to radiation in the RF range.
exposure, repeated This term refers to exposures lasting from a few
minutes to a few hours repeated on successive days.
exposure, short-term This term covers exposures lasting from a few
hours to 24 hours, or exposures for a few hours per day repeated
for a few days per week.
exposure, single This term usually refers to an uninterrupted
short-term exposure.
non-thermal effect See athermal effect. radiofrequency In the present document, this term is used to
designate frequencies ranging from 100 kHz to 300 MHz.
thermal effect An effect resulting predominantly from a local or
whole body temperature rise in the living organism.